Plasma motilin levels were measured by radioimmunoassay both pre- and postoperatively in 37 patients who underwent abdominal surgery. In 8 colorectal cancer patients with tumor removal and in 13 stomach cancer patients with total gastrectomy by Roux-en-Y anastomosis or subtotal gastrectomy of Billroth I anastomosis, the motilin levels decreased before the onset of postoperative peristalsis, and at the time when peristaltic sounds could be detected stethoscopically, the plasma motilin levels had increased by 200% of the preoperative level. Similar motilin levels were maintained until the time of the initial postoperative flatus and/or stool. Thereafter, plasma motilin levels decreased and returned to almost the same levels as the preoperative ones. The postoperative time course of plasma motilin in 10 gallstone patients was very similar to that in the 21 gastrointestinal cancer patients above. In 5 stomach cancer patients with subtotal gastrectomy of Billroth II anastomosis, however, the peak at the time of the initial postoperative peristalsis was not as remarkable as that in the 13 stomach cancer patients.