We previously found that local O 2 extraction efficacy in isolated pump-perfused intestine was enhanced when systemic reflex vasoconstriction was stimulated by hypovolemia (Samsel, R.W., and P.T. Schumacker. 1994. J. Appl. Physiol. 77: 2291-2298) . The microvascular mechanism underlying this beneficial effect could involve a redistribution of flow between mucosa and serosa, or an adjustment in the heterogeneity of perfusion within those regions. We measured regional blood flows and distributions of flow and capillary erythrocyte transit times in two segments of small intestine in anesthetized dogs ( n ϭ 10). Each vascularly isolated segment of intestine was pump-perfused under high flow (O 2 supply-independent VO 2 ) and low flow (O 2 supply-dependent) conditions. During the first gut segment, the animal was kept normovolemic using i.v. fluids to minimize reflex vasoconstriction. During the second, the animal was hemorrhaged to augment vasoconstriction ( n ϭ 7), or kept normovolemic to control for the effects of time ( n ϭ 3). Blood flow distributions were measured using 15 m radiolabeled microspheres. Tissue blood volume was measured using