Responding for intravenous injections of morphine was studied using a discrete-trials procedure in squirrel monkeys. For one group, each session consisted of 10 trials at an inter-trial interval of 15 min; for the second group, each session consisted of 100 trials at an inter-trial interval of 1.5 min. When different injection doses of morphine (1–1000 μg/kg per injection), including saline as a control procedure, were substituted at random for blocks of five consecutive sessions, the frequency of morphine self-administration was found to be an inverted U-shaped function of the injection dose. This relationship was observed in each group of monkeys, despite a 10-fold difference in the total amount of the drug which was available for self-administration per session when a given injection dose was substituted for both groups. The results show that the injection dose of morphine acted as a primary determinant of response probability, even under circumstances in which trial spacing imposed a significant delay between consecutive opportunities for drug self-administration.
