NADPH diaphorase histochemistry was used in combination with axonal labelling techniques to determine if NADPH diaphorase is present in afferent and postganglionic efferent pathways to the urinary bladder of the rat. In the L6 and S1 dorsal root ganglia, 80.9 and 78.5%, respectively, of bladder afferent neurons labelled with fluorescent dyes were NADPH diaphorase positive. In the major pelvic ganglion (MPG), many non-labelled neurons and fibers were intensely stained for NADPH diaphorase. Intensely stained cells were clustered near the exit of the penile nerve although stained cells were also scattered throughout the ganglion. Only a small percentage (3.5%) of bladder postganglionic neurons in the MPG were NADPH diaphorase positive. Since NADPH diaphorase activity commonly reflects the presence of nitric oxide synthase, the present findings raise the possibility that nitric oxide may have a role as a neurotransmitter or neuromodulator in afferent pathways from the urinary bladder.