Radial distension of the large intestine produced a slow depolarization in a population of neurons in the inferior mesenteric ganglion of the guinea pig. The slow potentials often occurred simultaneously with cholinergic fast potentials [( excitatory postsynaptic potentials (EPSPs]) yet persisted in the presence of nicotinic and muscarinic cholinergic antagonists when all fast EPSPs were absent. The amplitude of the distension-induced noncholinergic slow depolarization increased with increasing distension pressure. For distensions of 1-min duration at pressures of 10-20 cm of water, the mean depolarization amplitude was 3.4 mV. The slow depolarization was associated with an increase in membrane resistance, and prolonged periods of colon distension resulted in a tachyphylaxis of the depolarization. Desensitization of ganglion cells to the peptide substance P attenuated the distension-induced slow potential by an average of 49% +/- 17%. Thus, two colonic mechanosensory afferent pathways converge on principal ganglion cells in the inferior mesenteric ganglion: one was previously described to be mediated by acetylcholine, and the other is described here, whose transmitter remains to be determined but which preliminary evidence suggests is mediated in part by substance P. The noncholinergic afferent pathway may enhance the intestinal inhibitory reflex mediated by cholinergic mechanosensory afferent input to the abdominal prevertebral sympathetic ganglia.