Plants of the genus Senna that contain anthranoides derivatives are frequently used as cathartics. Radiological studies have demonstrated that patients with chronic constipation who have used stimulant laxative have colonic redundancy and dilatation more frequently than patients who have not. The objective of the present work was to study morphological and histochemical changes of the lower gut after administration of Senna occidentalis seeds for a long period to rats, as observed in skeletal muscle fibers. Fragments of the lower gut of young and adult rats treated with S. occidentalis seeds (2% for 171 days and 3% for 61 days in the diet) were submitted to histological and histochemical analysis and to densitometry. The most important finding was decreased oxidative enzyme activity in smooth muscle cells and in myenteric neurons of the large bowel. As oxidative metabolism is essential for ATP and energy production, these results suggest that the functional intestinal disturbance caused by the chronic use of Senna occidentalis as a laxative can be due to a metabolic effect involving energy production, which would decrease colonic motility and cause functional colonic dilatation, but without any irreversible anatomic change.