Bilateral radiofrequency lesions were made in the medial preoptic-anterior hypothalamic (MP-AH) area of 6 adult male rhesus monkeys; 5 sham-lesioned subjects served as controls. Behavioral analysis consisted of observations on copulatory behavior, yawning, masturbation and some aspects of social behavior. MP-AH lesions reduced or completely eliminated the display of manual contacts of the partner, mounts, intromissions and ejaculations without interfering with masturbation. Yawning, a sexually dimorphic behavior, was not affected either, Measure of several social behaviors indicated no evidence of social withdrawal or other aberrance of social interactions, which might have led to the decline in heterosexual behavior. The results with regard to copulatory behavior were consistent with the effects of MP-AH lesions in rats, cats and dogs. In rhesus monkeys it appears as though the MP-AH region is specifically involved in the mediation of heterosexual copulation and is not vital to the performance of other forms of male sexual activity such as masturbation. Also the MP-AH is not critical for the display of all sexually dimorphic behaviors. The types of behavioral change in MP-AH lesioned subjects differed to some extent from those following castration, indicating that the effects of the lesions cannot be explained as basically that of functional castration.