Director Irwin Allen wanted to use stop-motion dinosaurs for this film, but due to budget reasons he had to use lizards - mainly monitor lizards - as dinosaurs. Plastic horns and spikes were attached to them to make them look more like dinosaurs.
Irwin Allen wanted stop motion for the special effects but the film's budget precluded that so they were forced to work with lizards and other reptiles. 20th Century Fox had no option but to slash the budgets of all their feature productions at the time as the costs over Cleopatra (1963) were starting to spiral out of control.
One of the last screen credits for Willis H. O'Brien who was the mastermind behind the special effects for the original King Kong (1933). O'Brien's input was largely restricted to hundreds of conceptual sketches for the dinosaurs. Budget limitations meant that none of them were realized on film.
David Hedison was reluctant about making the film as he didn't think the material was any good. Seeing Jill St. John dressed in a pink outfit with a poodle on set didn't do much to make him think differently. Nevertheless, he applied himself anyway, to the extent that Irwin Allen offered him the lead in his next film, Voyage to the Bottom of the Sea (1961). Hedison turned it down but would later take a part in Allen's TV series based on the submarine film.
Universal planned to remake "The Lost World" in the U. K. with Jack Arnold directing, a project the director wanted badly to do, but according to friend and former star John Agar, the studio canceled the project because of heath issues with Arnold.