The reinforcing and discriminative stimulus properties of two met-enkephalin analogues (FK 33-824 and FW 34-569) were compared to those of morphine and other morphine-like opioids in rhesus monkeys. Intravenous injection of the two met-enkephalin analogues maintained response rates lower that those maintained by either morphine or codeine. This differences was not due to a lack of similarity between the interoceptive stimulus effects of the enkephalins and those of morphine-like opioids, since the enkephalin analogues were found to share discriminative stimulus effects with morphine, codeine, and etorphine. FK 33-824 and FW 34-569, however, had a slower onset of discriminative effect than either morphine or codeine. This slow onset of effect may account for the limited reinforcing effect of these enkephalin analogues.