There have been descriptions of a variety of abnormalities in motor function in patients with megacolon; however, colonic tone in megacolon has not been measured to date. Recent data suggest that colonic hypertonicity of carcinoid diarrhea is measurable with an electronic barostat. Using a barostat method, we have evaluated colonic transit, motility, and tone in a patient with severe constipation and localized chronic megacolon. Transit measurements showed a significant delay beyond the mid transverse colon; motility studies in the dilated region of colon showed abnormally low tone but normal phasic contractility, as measured by the barostat. In contrast, manometry showed low amplitude contractions relative to control data; this discrepancy probably results from the large capacity of the segment of colon examined. Chronic megacolon can be associated with abnormal colonic tone, which can be measured by means of an electronic barostat.