Adenosine is a potent vasodilator of the systemic circulation. Infusion of adenosine into the aorta causes water and sodium retention and a fall in glomerular filtration rate and renal blood flow. The effect of adenosine on medullary blood flow is unknown. Because systemic vasodilatory effects may confound its renal actions, adenosine was infused into the renal artery of anesthetized Munich-Wistar rats at doses of 2, 6, and 15 micrograms/min. A marked dose-dependent increase in urinary flow and sodium excretion was observed. Inulin and p-aminohippuric acid clearance did not change significantly. Blood flow in vasa recta in the exposed renal papilla, as determined by fluorescence videomicroscopy, increased significantly only with the highest dose of adenosine. In control animals infused with the vehicle only, there was no change in any of the above variables. These results indicate that direct intrarenal infusion of adenosine in the rat increases urinary flow and sodium excretion and at higher doses also increases vasa recta blood flow. The effects on urinary flow and sodium excretion were therefore mediated by a mechanism other than an increase in vasa recta blood flow.