Because producer Dick Randall was simultaneously making kung-fu films in Rome, a cameo for a Bruce Lee imitator, Bruce Le, was written into this film, even though this scene makes no sense in the context of the rest of the film.
Most of the guts and gore were actually internal organs of dead animals, with real blood taken from a slaughterhouse.
(at around 1h 1 min) A pig carcass was used for the close-up of the chain saw cutting through the tennis player.
During the scene with the women in the dance class, director Juan Piquer Simon originally wanted them all to be naked. But Carmen Aguado, who wasn't an actress, rather an actual aerobics instructor they hired for the part, refused and informed him that dance classes weren't usually done in the nude as the dancers always wore tights. Simon wasn't happy about it, but backed down, even though a few of the extras playing the dance students said they'd be willing to do it naked. Simon reportedly took down their names and used them for nude scenes in later films.
Filming commenced in the winter of 1981. The pool scene had the actress swim in freezing cold water, and she almost froze to death as a result.