The pontomedullary raphe magnus (RM) contains two physiologically defined types of neurons that participate in the opioid-induced modulation of dorsal horn nociceptive messages: OFF cells, which decrease, and ON cells, which increase their discharge rates when reflex behavior is evoked by noxious pinch or heat. Because both types of neuron have inhibitory inputs and because there is evidence that gamma-aminobutyric acid (GABA) inhibitory mechanisms within RM contribute to the antinociceptive action of opioids, we have sought anatomical evidence for a direct GABAergic input to OFF and ON cells. In this study, cells of each type located in the RM were electrophysiologically defined and intracellularly filled with horseradish peroxidase or Neurobiotin. One cell of each type was labeled in the cat, and 2-3 cells of each type were labeled in the rat. Thin sections were labeled by a postembedding immunogold procedure by using an antibody directed against glutaraldehyde-conjugated GABA. GABA-immunoreactive (GABA-ir) boutons contained small, round, clear vesicles and made symmetrical synapses with identified dendrites. GABA-ir boutons were apposed to soma and to proximal and distal dendrites of both cell types in both species. These findings demonstrate direct GABAergic input to identified OFF and ON cells in the RM. J. Comp. Neurol. 378:196-204, 1997.