The effect of prior administration of reserpine on fentanyl dose-response curves for loss of the righting reflex and prevention of purposeful movement response to noxious stimulation was studied in rats. It was found that reserpine (5 mg · kg−1, 3 h before the tests) antagonized the effect of fentanyl on purposeful movement response to the tail clamp and, at the same time, strengthened its effect on the righting reflex. As a result, reserpine pretreatment reversed the order of fentanyl potency regarding these two effects. Reserpine changed fentanyl ED50values for the purposeful movement response from 8.2 to 20.3 μg · kg−1(P< 0.0001) and for the righting reflex from 20.5 to 13.7 μg · kg−1(P< 0.0001). The results suggest that reserpine dissociates the analgesic action of fentanyl from its anesthetic action, defined as a loss of the righting reflex. This may be regarded as an indication that the analgesic action of narcotics may not adequately reflect their anesthetic potency.