Miklós Rózsa's score is one of the few movie scores to make extensive use of the cymbalum (a hammered dulcimer-like instrument). The soundtrack memorably features a beating heart to signal the mind-control attempts and eerie music from a cymbalum accompanying the film's more suspenseful moments. The instrument can be seen being played at the beginning of the film.
This was Byron Haskin's last feature film as director, and the final film that George Pal produced under his contract with MGM. According to Haskin, the studio was so anxious to be rid of Pal that they deliberately sabotaged this film, casting it with the wrong actors, keeping the budget too low and skimping on the all-important special effects.
MGM announced plans to produce the movie with George Pal as producer/director during its 40th anniversary celebration in 1964. Unlike many of the announced projects, this one actually got made but wasn't released until 1968.
Miklós Rózsa's first film score for nearly five years. Although he lived into the mid-1990s, he did only a handful of film scores subsequently.
While Jim Tanner is at the hotel lobby newsstand during a convention on the magazine rack is seen the July 1966 EERIE Comic Book issue #10.