Reflex discharges of pelvic postganglionic parasympathetic efferent fibers on the bladder surface induced by afferent volleys in the hindlimb nerve have been recorded in anesthetized rats, and the effects of the nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME) on the reflex discharges have been investigated. Single electrical stimulation of the tibial nerve at intensities supramaximal for excitation of A- and C-afferents evoked a reflex discharge in the postganglionic parasympathetic efferents with four distinct components, i.e., two inhibitory components with latencies of 49 and 203 ms, respectively, and two excitatory components with latencies of 126 and 308 ms, respectively. These reflexes could be observed when the bladder was expanded, but not markedly when the bladder was empty. Intravenous administration of L-NAME resulted in (a) a reduction in the level of resting discharge, (b) a reduction in the size of the first inhibitory component, (c) the disappearance of the second inhibitory component and (d) the exaggeration of the late excitatory component. Intracisternal injection of L-NAME caused changes similar to those observed following intravenous injection. The results suggest that inhibitory components of the somato-pelvic parasympathetic reflex are mediated by pathways that utilize nitric oxide as a neurotransmitter or neuromodulator at the level of the brainstem.