The antinociceptive effectiveness of spinally administered opiates raises the question of their neural target in the spinal cord. A large body of evidence points to a site of action on primary afferent fibers before the first central synapse. The dorsal root potential (DRP) is a consequence of phasic presynaptic inhibitory action on the primary afferent fibers. In these studies in the rat, dorsal root potentials, evoked by non-noxious subcutaneous electrical stimulation, were altered by systemic and spinal administration of opiates and by two types of noxious stimulation. Noxious stimulation was accompanied by increased dorsal root potential amplitude, whereas antinociception was associated with decreased dorsal root potential amplitude. Treatment with morphine decreased the dorsal root potential and blocked the increase in amplitude induced by noxious stimuli. These results indicate that nociception and antinociception differentially modulate the phasic component of presynaptic inhibition on primary afferent fibers in the spinal cord.