In male rats copulation has antinociceptive effects as measured either by shock-induced vocalizations or hindlimb withdrawal to pinch. Prolonged mating reduces the content of endogenous opioids in midbrain but not in hypothalamus or caudate nucleus. Blockage of opiate receptors with the narcotic antagonist naloxone (4 mg/kg) significantly extends the postejaculatory interval. The results indicate that mating is a biological stimulus for the release of endogenous opoids, possibly to (a) prevent intense sexual stimulation from becoming aversive, and (b) increase its reward value.