Angiotensin II is more potent as a vasopressor than angiotensin III when given intravenously. We tested the hypothesis that differential changes in mesenteric blood flow contribute to this difference in potency. The effects of angiotensin II and angiotensin III on mesenteric blood flow were compared in 31 dogs anesthetized with pentobarbital. These agonists were administered either as bolus injections (10-160 pmol) or as constant infusions (30 pmol/min per kg) directly into the vasculature supplied by the superior mesenteric artery. Approximately equipressor doses of angiotensin II (40 pmol/min per kg) and angiotensin III (80 pmol/min per kg) also were given intravenously. On the basis of duration and graphic integration of the response in mesenteric blood flow, but not on the basis of absolute change in amplitude, angiotensin II was consistently more potent than angiotensin III as a mesenteric vasoconstrictor. The intra-arterial and intravenous constant infusion doses were repeated after the administration of meclofenamate (4 mg/kg, iv). Meclofenamate did not alter any of the responses to angiotensin II or angiotensin III. We conclude that the differential constrictor properties of these compounds in the mesenteric vasculature contribute to the greater potency of intravenously administered angiotensin II on arterial pressure. The results provide no evidence for an interaction between prostaglandins and the vasoconstrictor properties of angiotensin II or III in the intact mesenteric vasculature of the anesthetized dog. Circ Res 46:146-151, 1980