In an effort to evaluate the "spillover method" of measuring blood flow during angiography, we explored the effects of rapid injection of diatrizoate meglumine and diatrizoate sodium (Renografin-76) at rates no greater than resting blood flow in the superior mesenteric artery of anesthetized dogs. An increase in mesenteric blood flow and a decrease in mesenteric arterial pressure occurred many seconds after cessation of the injection. The dilation effected by the contrast agent was less marked at elevated blood flow values than at resting flows. The pattern of response to diatrizoate meglumine and diatrizoate sodium resembled that observed with vasodilator drugs. Our findings also suggest that the initial vascular response to a pulsation of the diatrizoate medium is vasodilation.