The resting motility of the pelvic colon was studied in 28 patients with constipation and compared with control subjects and patients with diarrhea. Colonic activity in patients who had been shown to have slow colonic transit was not significantly different from controls. In contrast, activity in patients who complained of constipation but who were found to have normal colonic transit time was increased (P less than 0.02). The response of the pelvic colon to the introduction of a surface-acting laxative was studied in 18 patients with slow-transit constipation. Eleven patients developed progressive peristaltic waves, while in 7 there was no response. It is possible that the relative inactivity of the colon in the latter group is due to a disorder of the myenteric plexus. If so, the bisacodyl stimulation test may distinguish patients with an abnormal myenteric plexus from those in whom it is normal.