The technique of measuring penile erection is the most valid assessment of sexual arousal in males (Zuckerman, 1971). The stimuli used during assessments of sexual preference with this technique have for the most part, been still or moving pictures of nude single females and males (Freund, 1963; McConaghy, 1967; Barlow, Becker, Leitenberg and Agras, 1970). Recently Abel, Barlow, Blanchard and Mavissakalian (in press) have shown that erotic films produce significantly greater penile circumference changes than either slides or audiotaped descriptions in homosexual males. Similarly, Sandford (1974) showed the superiority of films over slides in heterosexual males and suggested that sexual activity rather than nakedness per se may be the more important in producing sexual arousal. The main purpose of this study was to determine what sexual activities produce significantly different sexual responses in homosexual and heterosexual males.