Histamine release adjacent to mesenteric arterioles under conditions of ischemia causes vasodilation and increases regional blood flow. This is presumably a protective mechanism which may be blocked by the use of H1 and H2 antagonists. Mesenteric arterioles of 29 rats were observed with intravital microscopy. Diameter and erythrocyte velocity were measured and arteriolar flow was calculated. Histamine was topically applied to the vessels under view in sequentially increasing concentrations (10(-7) to 10(-4) M). Following systemic injection of an H1 or H2 receptor antagonist or ibuprofen, the application of 10(-4) M histamine was repeated. Topical histamine caused vasodilation (122% of control; P less than 0.05) at 10(-4) M with a corresponding increase in erythrocyte velocity and calculated flow (118 and 177% of control, respectively; P less than 0.05 for each). The vasodilatory effects of histamine were blocked by systemic injection of histamine receptor antagonists (H1 + H2 greater than H1 greater than H2), while ibuprofen had no significant effect. In situations in which the gut is at risk for ischemia, the use of H1 and/or H2 receptor antagonists may seriously compromise the mesenteric microcirculation.