A single injection of morphine or isotonic saline was administered to rats at either 1, 5, 9, ro 13 days of age. At 26-29 days of age, animals were tested for pain responsivity and activity following either morphine (5 mg/kg) or saline. Animals which received morphine at 1 day of age displayed tolerance at 26 days of age, whereas animals which received morphine at either 5, 9, or 13 days of age failed to display tolerance. This age-dependent change in susceptibility to single-dose morphine tolerance may involve alterations in opiate receptor ontogeny.