Penile volume and galvanic skin responses to nude female and male film sequences were studied in 10 transsexual patients, 44 patients requesting treatment for homosexual impulses, and 60 heterosexual students. Student controls and homosexuals showed significantly greater galvanic skin responses to the preferred than to the nonpreferred sex. Transsexuals tended to show larger galvanic skin responses to females than did male homosexuals. No strong relationships were found between penile volume and galvanic skin response to the preferred sex. It is concluded that transsexual patients differ significantly from homosexual patients in autonomic responsivity, which may have diagnostic usefulness.