Intravenous injections of a synthetic enkephalin analog, FK-33-824 were found to maintain operant responding under second-order schedule control (FR 4[VR 16:S]) in five, morphine-dependent rhesus monkeys. All monkeys self-administered enkephalin in amounts equivalent to morphine at comparable doses per injection. Each of five doses of enkephalin (0.5, 0.25, 0.125, 0.05 and 0.01 mg/kg/inj) were substituted for morphine during 10 consecutive sessions over a 56 hr period. No monkey developed opiate withdrawal signs during enkephalin substitution except at the lowest enkephalin dose (0.01 mg/kg/inj). Although the number of enkephalin injections self-administered increased as the dose per injection progressively decreased, there was a significant linear decrease (p less than 0.05) in mg/kg/enkephalin per session at doses of 0.25 mg/kg/inj and below. Reductions in morphine dose per injection, over a range of 0.5 to 0.125 mg/kg/inj produced comparable increases in number of injections per session, but no significant changes in morphine intake. The number of food pellets earned on a second order FR 4 (VR 16:S) schedule decreased during enkephalin substitution. These decreases were significant at the highest doses of enkephalin (0.5 to 0.125 mg/kg/inj). These data attest to the reinforcing characteristics of an enkephalin analog in rhesus monkey and suggest that natural polypeptides may contribute to the reinforcing properties of opiate drugs.