The profile of secretin release along the entire canine small intestine was examined in this study. Four equal loops of the small gut, from the duodenal bulb to the ileocoecal valve, were isolated. In eight anesthetized dogs the four segments were perfused for 40 min each in random order with an acidified (pH 2.5) emulsion of 20 mM oleic acid. In four dogs control experiments were performed using 0.15 M saline. Secretin release in portal venous blood was measured by a sensitive radioimmunoassay. Although secretin was mainly released in the first quarter of the small intestine (310 pM X 40 min), large amounts of secretin, 33% of the total secretin release, were liberated in the second quarter of the small intestine (164 pM X 40 min). Minute amounts of secretin (23 pM X 40 min) were released in the third quarter, whereas perfusion of the last quarter of the small gut failed to release secretin. We conclude that the major portion of secretin is releasable in the first quarter of the small gut. High amounts of secretin can be liberated in the second quarter of intestine, an area that is probably never exposed to pH below 4.5 (the known threshold for secretin release by acid), but is still exposed to fatty acids (other releasers of secretin).