Recent reports of opiate receptors in the peripheral nervous system have led to the hypothesis that the analgesic action of opiates might, in part, result from a reduction in response of peripheral nerve fibers thought to be concerned with signaling pain (nociceptive afferents). The authors examined the effects of the narcotics, fentanyl (up to 30 μg/kg, iv) and morphine (1 mg/kg, iv), on the response of single unmyelinated afferents (C-fiber nociceptors and warm fibers), recorded a monkeys, to heat stimuli applied to their receptive fields. Neither narcotic affected the response of the afferents. In addition, naloxonc did not affect their response. Thus, an alteration of cutaneous nociceptor response is unlikely to contribute to the analgesic action of narcotics.