Cholecystokinin-like activity in the duodenal mucosa was measured by the bioassay method described by Ljungberg to elucidate its significance in 14 duodenal ulcer patients as well as in 13 normal subjects with no evidence of gastrointestinal diseases. The stage of duodenal ulceration was determined endoscopically according to the criterion of the Japanese Gastroenterological Endoscopic Society. The cholecystokinin-like activity in the duodenal mucosa of duodenal ulcer patients in active stage 1, which was considered as an early stage of active open duodenal ulceration, did not differ statistically from that of normal subjects, whereas that of duodenal ulcer patients in active stage 2 began to show a significant increase (p less than 0.05), and the cholecystokinin-like activity in the duodenal mucosa of duodenal ulcer patients in healing stage 1 or healing stage 2 was significantly higher than that in normal subjects (p less than 0.01). The cholecystokinin-like activity in the duodenal mucosa of duodenal ulcer patients in the scarring stage, however, returned to the normal range. It is concluded that cholecystokinin may act physiologically in the cure of duodenal ulcer.