Rats administered 5 mg/kg morphine SO4, through subcutaneously implanted catheters, during each of several daily sessions in an open field showed a progressive increase in locomotor activity measured in the open field prior to each morphine administration. Since the increases in activity were not observed in rats given morphine in a different environment (home cage) and saline in the open field, it is concluded that the increases were due to conditioning. In addition, the increases in activity were retained over a 7-day rest period; they were also produced when a second opiate (5 Mg/kg etorphine HC1) was substituted for morphine, were not seen when 2 mg/kg naloxone HC1 (lp) was administered during treatment, and were present in rats showing tolerance to opiateproduced hypoactivity. Morphine's direct effect on activity is believed to have a biphasic dose-response curve; therefore, the relation of dose to conditioning was also studied. Increases in activity were the only conditioned behaviors observed; they were present only at the higher doses (16, 4, and 1 vs. .25, .065, and 0 mg/kg), and they occurred whether or not the dose was associated with unconditioned hypoactivity. The discussion deals with the relation of conditioning and morphine tolerance, the question of whether the unconditioned stimulus of morphine conditioning is a compensatory or a direct effect of morphine, and the similarity of conditioned increases in activity produced by morphine and by other stimuli that are reinforcing.Recently, it was demonstrated that morphine tolerance in intact rats involves classical conditioning. Siegel (1975) showed that embedded within procedures commonly used to produce tolerance is a classical conditioning procedure, with morphine effect constituting the unconditioned stimulus Some of the data were reported at the Canadian Psychologists Association Meeting held in Ottawa, Canada, in June 1978. The authors acknowledge the typing assistance of June Shepperd and Val Cabral, and the valuable comments of Barbara Hering, Derek van der Kooy, and Constantine Poulos on earlier drafts of the manuscript.