Length of the various parts in the inferior mesenteric arterial and venous system was measured in 120 cadavers of the Japanese from the view-point of the possible limit of direct rectocolostomy in rectectomy. 1) When the blood supply from the origin of the inferior mesenteric artery (O), through its first branch (P), then the lower branch of the left colic artery or the sigmoid colic artery which is the branch of the left colic artery, to the lowest end of the marginal artery of the desce ding colon (S) in utilized for the direct rectocolostomy, it is necessary in more than half of the cases to have the sigmoid colon remained for the length of 15 cm to 20 cm below the lowest end of the descending colon. 2) When the blood supply from O, through the highest end of the upper branch of the left colic artery (C), to S, i.e. OPCS is utilized for the purpose, it is sufficient enough in most of the cases to have the sigmoid colon remained for the length of only 5 cm. 3) When the inferior mesenteric artery is cut and the blood supply from only the middle colic artery is utilized, the direct rectocolostomy is possible in most of the cases. 4) For the direct rectocolostomy, the inferior mesenteric vein should be cut.