Parasympathetic preganglionic discharges recorded from pelvic nerves in spinal cats were evoked by single-pulse stimulation of sacral afferent fibers of descending excitatory pathways in the thoracic spinal cord. Evoked responses were analyzed on-line by signal averaging. Discharges evoked by either pathway were increased in size by 2-10 times by coadministration of both picrotoxin and strychnine, but not by either drug alone. The combination also markedly enhanced evoked increases in bladder pressure. Strychnine was also effective with bicuculline and in cats depleted of central GABA stores by semicarbazide. The monoamine precursors, 5-HTP and L-dopa, only depressed evoked discharges in both untreated and convulsant-treated animals; this depression was enhanced by tricyclic antidepressants. The results indicate that sacral parasympathetic preganglionic neurons are controlled by an unusual type of strong, local tonic inhibition which is mediated by both GABA and glycine. Both amino acid transmitters must be blocked to unmask this inhibition. Interruption of supraspinal control of this local inhibition may account in part for loss of bladder function following spinal injuries. The 5-HT and NE bulbospinal pathways that terminate near the preganglionic neurons also appear to be inhibitory. Enhancement of this inhibitory monoaminergic transmission by tricyclic antidepressants may contribute to the efficacy of these drugs in treating nocturnal enuresis.