Electrical activity and synaptic responses were recorded intracellularly in 415 neurons of the mouse superior mesenteric ganglion (SMG) attached to a segment of distal colon in vitro. Eighty-seven percent of neurons tested received ongoing nicotinic cholinergic fast excitatory postsynaptic potentials (fEPSPs). Colonic distension caused an initial transient followed by a sustained, slowly adapting increase in fEPSP activity. Application of hexamethonium only to the colon reduced, but did not completely abolish, distension-evoked responses, suggesting direct projection of some distension-sensitive fibers. Ongoing fEPSPs were abolished when nerve trunks connecting the SMG to the colon were transected or blocked with tetrodotoxin applied to the colon. Intracellular labeling with horseradish peroxidase or lucifer yellow revealed that about 90% of neurons receiving colonic synaptic input had a caudally projecting axon; about 60% that did not receive colonic input had a rostrally projecting axon. The latter neurons were found only in the cephalad ganglion region. These results show that mouse SMG neurons receive colonic mechanosensory afferent synaptic input and thus may participate in sympathetic intestinal reflexes.