In 20th century New Mexico, a group of cowboys are struggling to save their failing rodeo, The Breckenridge Show. Champ then leaves the arena for a short break, where he bumps into "Rowdy" Gonzalez (Dennis Kilbane) and "Bean" Gonzalez (Mario De Barros) on his way out. After they escape Gwangi, the posse find a small cave perched on a ledge of rocks to hide in. When Champ goes to look through it, he sees someone else in the corner (The aforementioned person is Peso, a very starved and disheveled man played by Lloyd Bridges). Carlos goes to impale Gwangi with a giant fence pole, but is knocked over. This causes the metal rod to also knock over a lit candle, setting the area around Gwangi on fire. Carlos very narrowly escapes an leaves Gwangi to burn in the corner. He, T.J., Rowdy, Bean, and Clyde manage to get out of the church as it is slowly engulfed by flames. The structure of the building becomes so unstable that Gwangi, who is at this point on the top floor, falls through the back of the church and tumbles off the cliff behind the building, getting impaled on the rocks below and bleeding to death. His lifeless pelt however continues to burn as it is still on fire. The film ends with the whole town watching the church burn to the ground.
- Gustavo Rojo (Carlos Dos Orsos)
- Gila Golan (T.J. Breckenridge)
- Dennis Kilbane ("Rowdy" Gonzalez)
- Mario De Barros ("Bean" Gonzalez)
- Warren Beatty (Clyde Connors)
- Curtis Arden (Lope)
- Lloyd Bridges (Peso)
- Richard Carlson (David "Champ" Connors)
- James Franciscus (Randall "Tuck" Kirby)
- Laurence Naismith (Professor Horace Bromley)
- Freda Jackson (Tia Zorina)
- José Burgos (Zwerg Zorina)
- Richard Chamberlain (Orlando Zorina)
- Robert Rietty (Fernando and Miguel Zorina)
Gwangi was originally conceived by Willis O'Brien (1886-1962), the man who created the special effects for the original King Kong (1933). The plot was inspired by Sir Arthur Conan Doyle's book The Lost World (1912), with added elements from King Kong (capturing a monster and bringing it to civilisation where it runs amok). In O'Brien's scenario, then called Valley of the Mists, cowboys discover an Allosaurus in the Grand Canyon. After finally roping the dinosaur, they put it in a Wild West show, but the creature, now called Gwangi, breaks free and fights lions in the show that have also escaped. After killing the lions, Gwangi goes on a rampage around the town and is run off a cliff by a man in a truck. O'Brien died before The Valley of Gwangi was filmed. A similar film, The Beast of Hollow Mountain, appeared in 1956, produced by O'Brien and a Mexican film company with the same story. Harryhausen was not involved with the visual effects for that film.
Gwangi was described in O'Brien's original script as an 'Allosaurus', although O'Brien apparently didn't draw much distinction between Allosaurus and T. rex, as he also referred to the T. rex in the original King Kong (modeled by Marcel Delgado) as an allosaur. According to Ray Harryhausen, his own version of Gwangi (and O'Brien's Gwangi too, as well as Delgado's Kong T. rex) was based on a Charles Knight painting of a T. rex - one of the two most famous paintings by Knight, and one that is instantly recognizable by the eye being placed too far forward on the skull (this was based on concurrently incomplete skeletal remains and the eye was mistakenly placed in one of the nasal sockets), as well as incorrectly portraying T. rex with a three-fingered hand. This famous T. rex image is also reflected in Harryhausen's Rhedosaurus in 'The Beast From 20,000 Fathoms', and even in the 2005 King Kong's 'Vastatosaurus'. In a DVD interview Harryhausen said "We sometimes called it an 'Allosaurus'... They're both meat eaters, they're both Tyrants... one was just a bit larger than the other." Gwangi was scaled to be 14 feet tall, within the size range of an average adult T. rex (although not the largest), and at the upper limit of the largest Allosaurus specimens.
The Valley of Gwangi was the last dinosaur-themed film that Harryhausen animated, and he made much use of his experience in depicting extinct animals from his earlier films. Close to a year was spent on the special effects (there were over 300 'Dynamation' cuts in the film, a record number for Harryhausen), with the roping of Gwangi being the most labour-intensive animated sequence. It was achieved by having the actors hold on to ropes tied to a "monster stick" that was in the back of a Jeep. The jeep and stick when filmed with Gwangi are on a back rear projection plate and hidden by his body, and the portions of rope attached to his body are painted wires that are matched with the real ropes. The coordination of Gwangi's animation with live actors on horseback (and the horses appearing to react to Gwangi) was particularly difficult to film, and the source of an editorial lapse in a following scene. Gwangi bites through the ropes around his neck when first lassoed and later has his jaws roped together when unconscious. However, he is then shown being transported in a cart again held only by ropes around his neck but with jaws now un-bound.
The first animated sequence in the film is a diving act done by T.J. and her horse. Because it was decided that it was too risky to have a rider and horse jump off a 40-foot high platform into a tank of water, a model horse and rider were used. After tempting Gila Golan's horse to jump from a mock-up platform onto a trampoline, the film cut to an animated model suspended on wires (actually it was just a tiny toy horse and rider bought in a toyshop). The splash was real, triggered by an electric charge inside the tank.
After local Gypsies steal the Eohippus, it is released into the Forbidden Valley. For all the scenes where the cowboys are chasing the creature, the animated model was used. However, in one long shot, a baby goat was used instead because the model would have been too small.
The pterosaurs were mistakenly given bat's wings (with elongate fingers supporting the membrane; pterosaurs had one finger forming the wing's leading edge but none on the membrane). The wings appear to mimic those of a pterosaur from an earlier Harryhausen film, One Million Years BC (1966). Close-up sequences of the pterosaurs in Gwangi were provided by life-size models. For the scene when Lope is snatched from his horse by the Pteranodon, the boy was raised by wires painted out in the studio and Harryhausen animated the eight-inch high model pterodactyl to correspond with his movements. However, once the creature gets up to a certain altitude the real boy was replaced with a model which was used until he crawls away from the creature which is being killed by Carlos on the ground. Bromley the paleontologist mistakenly calls it a Pterodactyl (a common error) while he is inspecting it on the ground.
The scene where Gwangi pounces on the Ornithomimus has been copied many times in dinosaur movies, primarily in Jurassic Park, when the Tyrannosaurus Rex pounces on the raptors. The smaller dinosaur had never appeared on the movie screen before and although its movements were unlikely it was one of Harryhausen's favorite sequences in the film. The battle between Gwangi and the Styracosaurus features battle moves such as biting, stabbing, butting, and pinning. After Gwangi is captured, he is wheeled back to town in a cart and for several of the long shots of this scene the crew built and used a full-sized mock-up of Gwangi, again because an actual model would have been far too small.
Harryhausen originally planned to have used a real elephant in some of the scenes for the fight with Gwangi. This did not work out because he wanted to have used a 15-foot tall elephant (the world's biggest elephant was two feet shorter than this). So the live 8-foot elephant was only used in the beginning when T.J. is seen briefly riding on its back. For some of the elephant fight scenes Harryhausen used the animation table as the bullring floor.
For Gwangi's death scene a number of special effects were used. When the torch hits the ground near Gwangi, the flames are seen quickly developing and surrounding Gwangi. The flames were added in by double printing the camera. The outside of the burning church was a mixture of composites: the lower half was the real church photographed on location in Spain, and the upper, burning half was a miniature, again added in by double printing the camera.
The model of the Eohippus was supposed to have had toes but appears to have had regular hooves with 'toes' painted on (the sound effects of the animal moving also resemble hooves). The model of the Styracosaurus featured an inflatable air 'bladder' to simulate the animal breathing heavily after its combat with Gwangi (a feature first used in models made for much earlier films by Marcel Delgado).
Some of the models used in the film featured were reused model armatures from earlier films. Gwangi, the Ornithomimus, and the Styracosaurus were all made from the Ceratosaurus, the phororhacos, and the Triceratops, who were stripped down and had their armatures modified for further use. The actual model of Gwangi was about 12 inches high and the Ornithomimus was about 8 inches high. A solid-latex, non-armatured model of Gwangi was also used for the scenes when he knocks himself out while trying to exit the valley in pursuit of the cowboys (Harryhausen was never pleased with this, as the solid model didn't look right).
Actress Gila Golan's Israeli accent was so strong that all of her lines were redubbed on the film by a voice actress. Actor Laurence Naismith, who plays Professor Bromley, had earlier appeared in Harryhausen's Jason and the Argonauts as the shipbuilder Argos.
The movie was filmed in Almería and Cuenca, Spain. The bullring scenes were shot in Almería's plaza de toros and the finale at Cuenca's cathedral. The unusual rock formations of Ciudad Encantada near Cuenca were used for the forbidden valley.
By the time of the film's release, interest in 'monster' films of this type was waning. Management at Warner Brothers and Seven Arts also changed and the film was released with little promotional effort on a double-bill with a biker film; it thus missed its target audience and was not as successful as earlier Harryhausen efforts.
The scene where Gwangi suddenly appears from behind a hill and snatches a fleeing ornithomimus in his jaws was later copied in the big-budget dinosaur movie, Jurassic Park.
During the 1980s hit TV series Scarecrow and Mrs. King, anytime a television was shown on in the series, The Valley of Gwangi was on the screen.
Justin Parpan's 2006 children's read-aloud book, Gwango's Lonesome Trail (Red Cygnet Press, Inc., ISBN 1-60108-004-2) features a pre-historic dinosaur named "Gwango" roaming the contemporary American Southwest.
In an episode of the situation comedy Friends, Ross watches the movie while in a hospital.
In the 2011 animated movie, Scooby-Doo! Legend of the Phantosaur, during a night time chase scene through the town a movie theater can be seen in the background playing two dinosaur-themed monster movies,The Valley of Gwangi and The Beast from 20,000 Fathoms (another Harryhausen film from Warner Bros).
- The death of Peso was heavilly cut when the film was released to cinemas due to the shots of the horse riding through the valley with his severed legs on it's back and Gwangi swinging his torso around by his neck.
- Gustavo Rojo, Gila Golan, and Warren Beatty were voice dubbed by different actors.
- Gustavo Rojo was dubbed by Robert Rietty (the actor who played Miguel Zorina on-screen in this film).
- It is unknown who dubbed Gila Golan and Warren Beatty.
- Champ and Peso are played by two famous hollywood actors.
- Champ was played by Richard Carlson.
- Peso was played by Lloyd Bridges.
- Bean was played by Mario De Barros, who later starred in 'Flesh + Blood' (1985) by Paul VerHoven.