Discharges were recorded from spinal dorsal horn neurons in response to noxious skin heating (50 degrees C) in anesthetized cats. Repetitive electrical stimulation of the superficial peroneal (SP) and posterior tibial (PT) nerves inhibited these nociceptive discharges as well as systemic administration of met-enkephalinamide (synthetic endorphin). The inhibitory effects of electro-stimulation as well as administration of synthetic endorphin were partially but significantly antagonized by naloxone, thus arguing in favor of endogenous morphine release as the basis for the mechanism of acupuncture analgesia. However, non-responding neuronal units to the administration of enkephalin were also observed in the present study. These non-responding units may explain why acupuncture is not effective in some individuals.