The blood supply to the descending colon of the horse was studied by gross dissection and methyl methacrylate corrosion casts. The arterial supply is derived from the left colic artery and cranial rectal artery with the left colic artery supplying approximately the proximal three fourths. Each artery gives off four to eight arcuate arteries that form a series of anastomosing arcades. The arcade pattern continues to form a marginal artery that parallels the long axis of the colon. Small branches from the marginal artery anastomose with adjacent branches to form a secondary arcade. The secondary arcade lies approximately 1 cm proximal to the mesenteric teniae and sends off long arteries at regular intervals that course circumferentially around the bowel. The long arteries begin under the serosal mesothelium and perforate the muscular layers to course in the submucosal layer. The long arteries branch off supplying the wall of the colon and arborize over the antimesenteric surface, anastomosing with the vessels from the opposite side. The result is the formation of a series of vascular rings surrounding the colon. Venous return, in general, parallels the arterial supply.