Neuronal activity changes in the medial preoptic area (MPOA) of the male monkey were related to the commencement of sexual behavior, penile erection and the refractory period following ejaculation. Similarly, changes in the female MPOA were related to the commencement of sexual behavior and presentation. Increased neuronal activity in the dorsomedial hypothalamic nucleus (DMH) in the male monkey and in the ventromedial hypothalamic nucleus (VMH) in the female monkey was synchronized to each mating act. Stimulation study and neuronal activity recordings in the MPOA, DMH and VMH suggest involvement of MPOA neurons in sexual arousal, and of male DMH and female VMH neurons in the copulatory act. Stimulation experiment on the various parts in the hypothalamus of the female monkey also supports the conclusion.