The effects of naloxone on social play and feeding behaviors of postweanling and adult Long-Evans hooded rats were studied. Administration of 1.0, 5.0, or 10.0 mg/kg naloxone reduced play in a dose-related fashion. The effects of a 5.0-mg/kg dose of naloxone could be detected at least 3 h after administration, in terms of both a decrease in social behavior and decreased food consumption, indicating that the behavioral time course of naloxone effects is greater than some reports in the pharmacokinetic literature suggest.