The influence of melatonin on reproductive behavior was examined by administering it to pinealectomized male rats for 30 consecutive days. After the 30-d administration of melatonin or vehicle, each male rat was introduced into a female rat's cage on the day of proestrus and allowed to mate overnight. All rats administered the vehicle and the pinealectomized rats copulated; five of the twelve melatonin (8.0 mg/kg)-treated rats did not copulate. These findings suggest that melatonin inhibits the reproductive behavior of male rats.