Self-administration of morphine in the rat: relative influence of fixed ratio and time-out.

Rats were offered 3.2 mg/kg of morphine sulfate on a continuous reinforcement schedule until the daily injection rate had stabilized. The effect of fixed ratio schedules of 4 and 8 were compared to imposing time-out periods of 5 and 10 sec immediately followed each injection. The fixed ratio schedules decreased the injection rate while the time-out schedules had no effect. The hypothesis that the effect of a fixed ratio schedule is a consequence of imposing a time-out period, allowing full effects of the injection to be sensed, is not supported.