Colonic motility is known to be regulated, in part, by postganglionic sympathetic neurons whose cell bodies are located in the inferior mesenteric ganglion (IMG). Several studies have demonstrated that the neural activity of postganglionic neurons located in the IMG results from the synaptic integration of neural input received not only from the central nervous system via the inferior splanchnic nerves but also from sensory receptors located in the periphery and received via the lumbar colonic, intermesenteric, and hypogastric nerves. To investigate some of the interactions among colonic motility, peripheral neural input to the IMG, and postganglionic neural activity, intracellular recordings were made from single neurons located in the IMG of in vitro IMG-colon preparations obtained from guinea pigs. Recordings of colonic intraluminal pressures were simultaneously obtained with these neurophysiological recordings. Gross afferent input to the IMG from colonic mechanoreceptors was found to be nonadapting, to the correlated with physiological levels of intraluminal pressure, and to vary with propulsive motor activity. Both peripheral and central preganglionic inputs to the IMG reduced or abolished colonic motiligy, but with varying degrees of effectiveness. It is concluded that the firing patterns of sympathetic postganglionic neurons affecting motility of the terminal colon result primarily from the synaptic integration of inputs from preganglionic fibers of both central and peripheral origin.