Vaso-active intestinal polypeptide (VIP) and the related peptide, peptide histidine isoleucine, were infused intravenously in anaesthetized sheep. The VIP doses were designed to reproduce plasma concentrations seen after mesenteric ischaemia. The vasodilator action of VIP varied between different segments of the circulation and these differences in sensitivity were observed for both the degree and duration of the vaso-active action. A sustained vasodilation was detected in the coeliac artery and portal vein vascular beds during a 30-min VIP infusion. VIP is likely to be a contributory factor involved in the development of circulatory collapse during reperfusion after experimental mesenteric ischaemia.