The effect of indomethacin in hemorrhagic hypotension (HH) on total renal blood flow (RBF) and cortical flow distribution was studied in 9 pentobarbital anesthesized dogs. Local blood flow was measured as hydrogen clearance by 6 platinum electrodes in outer and inner cortex. RBF was recorded by electromagnetic flowmeter. Injection of indomethacin (3-5 mg/kg b.wt.) to inhibit prostaglandin synthesis, reduced renal blood flow only a few percent in control period. After the pretreatment with indomethacin, bleeding to mean arterial pressure 50 mmHg decreased RBF abruptly to 25% of control flow. Thus a renal vasoconstriction is observed both in early and late periods of bleeding in contrast to the initial vasodilation observed during the first half an hour of HH when prostaglandin formation was not blocked. Local blood flow in outer cortex decreased proportionally to flow in inner cortex during bleeding in both the indomethacin treated as in the untreated group, indicating an equal vasodilatory effect of prostaglandin in all parts of renal cortex during early periods of HH.