Tony Randall shaved his head for the role of Dr. Lao. It also made it easier to apply the make-up for the different characters he played. The studio publicity department wanted to photograph Randall getting his head shaved but arrived at the barber too late. They had the make-up artist glue hair back on Randall's head so Randall could pose for photos while getting the hair cut off again.
William Tuttle received an Honorary Academy Award for his make-up work on this movie. It was the first of two Honorary Oscars awarded for make-up - the other one being John Chambers for O Planeta dos Macacos (1968). It was not until 1982 that the Academy of Motion Picture Arts and Sciences decided to create a Best Make-Up category; Um Lobisomem Americano em Londres (1981) was the first film to win the award.
According to an article in "Starlog" magazine it took 11 days to stop-motion animate the segment where all the heads appear on the Loch Ness monster.
Although it is now regarded as a classic fantasy film, this was a box office disappointment when it was first released. It caused a four-year gap before George Pal had his next film in theatres. It also marked that last time that Pal would direct.