The plasma motilin levels were studied in seven patients with severe long-standing idiopathic constipation before and after a fat-rich test meal. The results were compared with those of a control group consisting of 10 apparently healthy persons with normal bowel function. The constipated patients had reduced basal motilin levels and a reduced motilin release after the test meal. Motilin is known to stimulate intestinal motility by inducing interdigestive descending contractions in the stomach and small intestine. A defective motilin release may therefore play a part in the pathogenesis of idiopathic constipation.