In this study, our aim was to test the hypothesis that colonic tone is abnormal in patients with irritable bowel syndrome (IBS). We studied eight patients with IBS and eight age-matched asymptomatic control subjects, in whom tone and motility were measured by an electronic barostat and by pneumohydraulic perfusion manometry, respectively. Tone and motility were recorded from the descending colon for a 14-hour period--3 hours awake, 7 hours asleep, 2 hours fasting after awakening, and 2 hours postprandially. In patients with IBS and in healthy subjects, colonic tone decreased by up to 50% during sleep and increased promptly on awakening. Fasting colonic tone (as quantified by the volume in the barostat balloon) in the awake state was not significantly higher in patients with IBS than it was in healthy subjects (125 +/- 13 versus 152 +/- 15 ml; P = 0.19). Tone increased postprandially in both study groups, and the increase was greater in healthy subjects than it was in patients with IBS (P < 0.05). The motility index during fasting was greater in patients with IBS than it was in healthy control subjects (3.2 +/- 0.6 versus 1.6 +/- 0.4; P = 0.05), and the postprandial increase in motility index was greater in the healthy subjects. Preprandially and postprandially, we noted a trend for high-amplitude prolonged contractions to be more frequent in patients with IBS than in healthy subjects. We conclude that colonic tone in patients with IBS showed the same nocturnal and postprandial variations as it did in healthy subjects.(ABSTRACT TRUNCATED AT 250 WORDS)