The odd symbol that appears again and again - on Wilbur's ring, on his grandfather's staff, in the design on the main floor, etc. - is actually an ancient Native American symbol commonly termed "Thunderbird in sun".
Two sound-stages were used for Sandra Dee's nude scene to prevent "peeping toms." One sound-stage was used to shoot the scene and the other was used to house the cast not needed in the shot.
"The Dunwich Horror" was one of the last on camera acting appearances by Ed Begley. This underrated character actor passed away in 1970, the year of the film's release.
AIP had previously wanted Mario Bava to direct an earlier, proposed version of this film, titled "Scarlet Friday", that would have starred Boris Karloff (with whom Bava had made Black Sabbath (1963)) and Christopher Lee (who had been the star of Bava's Hercules in the Haunted World (1961) and The Whip and the Body (1963)). The project was taken away from the director after the disastrous US theatrical run of Dr. Goldfoot and the Girl Bombs (1966).