Colonic smooth muscle spike potentials and contractility were recorded during the periods of stress by a bipolar electrode-perfused catheter apparatus placed in the rectosigmoid colon. Healthy subjects and patients with the irritable colon syndrome (ICS) were exposed to three standardized stressful conditions: (1) ice-water immersion, (2) Stroop stimulus differentiation test, and (3) ball sorting. In healthy controls, colonic motility increased after the first exposure to ice-water immersion (P less than 0.05), Stroop test (P less than 0.05), or ball sorting. Respiratory frequency also increased after exposure to the stressful stimuli. However, repeat exposures to the stress tests did not stimulate colonic motility. An increase in colonic motility occurred in patients with the irritable colon syndrome pretreated with a placebo after exposure to ice water (P less than 0.05), Stroop Test, or ball sorting (P less than 0.05). However, after exposure to the stressful situations patients pretreated with chlordiazepoxide had a diminished increase in colonic motility or in respiratory frequency. These studies suggest: (1) in healthy controls habituation reduces the stress-related increase in colonic motility, and (2) in patients with the irritable colon syndrome, chlordiazepoxide decreases the stress-related increase in colonic motility.