We examined the effects of combined lesions of the medial preoptic area and the medial amygdala on male copulatory behavior to understand a functional relation between these two structures in regulation of the behavior in the rat. Young adult male rats were castrated and received Silastic implants containing testosterone. After baseline copulatory tests, a unilateral medial preoptic lesion (uPOL) was made. Males with uPOL showed less intromission behavior than sham-operated males did, but none of them showed elimination of the behavior. Following the observations after the uPOL, these males were subjected to a unilateral medial amygdala lesion (uMAL) ipsilateral or contralateral to the previous uPOL. The ipsilateral uMAL did not affect copulatory behavior compared to that of sham-operated uPOL males. However, the copulatory behavior in the animals with uPOL was severely disrupted by the contralateral uMAL. These results suggest that the medial preoptic area is closely related to the medial amygdala in regulation of male rat copulatory behavior, and these areas may play a critical role in the behavior as an unitary system.