Percutaneous electrostimulation, acupuncture, and direct stimulation of the central nervous system are supposed to be capable of reducing painful sensation by releasing enkephalins and endorphins. We treated six volunteers with electrotherapy, obtaining in all cases a clear reduction of the pain induced by electric stimulation of the median nerve at the wrist. During the treatment, the administration of naloxone, an antagonist of morphine, in four subjects provoked a short but clear and immediate return of pain. In the other two cases, the drug provoked a further decrease of the painful sensation. During electro-stimulation in all patients, the somatosensory evoked potentials showed a statistically significant decrease. In the four "naloxone-responsive" subjects, the cortical evoked responses returned to basal amplitude after naloxone. In the two patients with a clinically paradoxic response, nonsignificant modification in the cortical evoked potentials was noted.