Recent interest in vascularity in relation to peptic ulceration poses the question of relationship between three entities: (1) site of gastro-jejunal (post-anastomotic) ulcer, (2) site of the anastomotic line and (3) site of junction and possible vascular anastomosis between gastric and jejunal circulations in the region of the anastomosis. Vascular injection studies in dogs show that the main gastric and jejunal circulations run up to, but do not communicate across, the surgical anastomosis, there being a line of least vascularity at the anastomosis. Since anastomotic ulcers occur distal to the anastomotic line, this supports the view that such ulcers are situated wholly in an area of jejunal circulation.