Pregnant Sprague-Dawley rats were daily administered either 20 mg/kg or naloxone HCl in saline or saline alone IP in two divided doses on days 7-20 of gestation (i.e., 40 mg/kg/day of naloxone). The offspring were reared by their biological dams and were examined for physical, neurological and behavioral development using a battery of psychoteratogenic tests administered from postnatal day 3 to adulthood (120 days of age). The Naloxone offspring were accelerated in postweaning growth, upper incisor eruption, righting development, startle development, home scent discrimination, and in directional swimming development, but as adults, showed impaired Biel water maze learning. The current data support the concept that early exposure of rats to naloxone can produce long-term changes in development and in adult behavioral performance.