This movie spent a long time on the shelf. Filming was completed in the early part of 1966, but its American release was not until late 1967, and its British one not until the spring of 1968. David Hemmings made this movie before his breakthrough role in Blow-Up (1966), and it is quite possible that the great (and unexpected) popularity of that movie was what finally pushed MGM into releasing this one. Many commented with surprise on the smallness of Hemmings' role - it is likely that his special billing, along with that of Sharon Tate, was an afterthought to disguise the fact that they had supporting parts. Although this movie was supposed to launch Tate, she had, because of its protracted shelf-life, already been seen in Don't Make Waves (1967), which she had made subsequently. That movie has a special "introducing" credit for her as a result.
Originally, Kim Novak was cast in the role of Catherine de Montfaucon under a three-picture deal with producer Martin Ransohoff, who had purchased the film rights to the novel as part of a multi-picture arrangement with MGM. Filming began in the fall of 1965 in France. Nearly every scene had been filmed when Novak fell from a horse and wasn't able to complete her scenes. Production was suspended for two weeks, but when Novak attempted to return she found she could not continue. She later said she had fractured a vertebra. Deborah Kerr was hired to take over and nearly every scene that featured Novak had to be re-shot with her replacement, save for a few long shots.
Screenwriter Robin Estridge also wrote the novel on which this movie was based. However, the novel was credited to his pseudonym, "Philip Loraine", while the movie script was credited to his real name.
This movie was filmed at Chateau de Hautefort in Dordogne, France.
Alex Sanders, an English occultist and Wiccan, was hired as a consultant to give the pagan rites some authenticity.