The authors studied the blood supply of the 3rd and 4th parts of the duodenum in 15 adult subjects. Although the duodeno-pancreatic arches are constantly found, the branches to the duodenum have a variable lay-out. On the right, the superior mesenteric artery, the distribution of which is always regular on both sides of the 3rd part of the duodenum. On the other hand, on the left, the superior mesenteric artery, there is sometimes a vascular hiatus which may extend to the duodeno-jejunal angle. Although in 60 p. cent of cases this part of the duodenum is well supplied with blood on both surfaces, in 40 p. cent of cases on the other hand, there exists an avascular area on both sides. This lay-out is not favourable for division and anastomsis of this segment of the digestive tract.