Neuronal activity changes in the medial preoptic area of the male monkey were related to the commencement of sexual behavior, penile erection and the refractory period following ejaculation. Increased neuronal activity in the dorsomedial hypothalamic nucleus was found to be synchronized to each mating act. The involvement of medial preoptic neurons in sexual arousal, initial penile erection and that of dorsomedial hypothalamic neurons in the copulatory act are suggested by the present findings.