Endothelium-derived prostaglandins appear to play an important role in myogenic contraction of the cerebral arteries in response to increased transmural pressure. The goal of this study was to determine whether prostaglandins contribute to the autoregulatory response of cerebral blood flow (CBF) during acute hypertension in normotensive and chronically hypertensive animals. In 4-5 months old Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) anesthetized with pentobarbital sodium (45 mg/kg, i.p.), regional CBF of the right parietal cortex was monitored by laser-Doppler flowmetry. Acute hypertension was induced by intravenous infusion of phenylephrine and the elevated arterial pressure was maintained for 50 seconds for the measurement of CBF. In the control group of WKY rats, CBF increased by 5.6% per 10-mmHg increase in arterial pressure. Changes in CBF were similar between the initial state (0-10 seconds) and the steady state (30-40 seconds) of acute hypertension. In WKY rats pretreated with indomethacin (10 mg/kg, i.v.), increases in CBF in the initial state were slightly but significantly greater than that in the steady state. Indomethacin also tended to enhance the increases in CBF in both the initial state and the steady state; however, the effect was not significant. In the control group and indomethacin-pretreated group of SHR, acute hypertension produced similar increases in CBF. In both groups, no significant difference between changes in CBF in the initial state and the steady state was observed. Thus our findings suggest that 1) in normotensive animals, the early autoregulatory response of CBF during acute hypertension is partially mediated by prostaglandins and 2) the role of prostaglandins in the regulation of CBF during acute hypertension is altered in chronically hypertensive animals.