The effect of captopril on cerebral blood flow (CBF) and its autoregulation was studied in spontaneously hypertensive (SHR), Wistar-Kyoto (WKY) and Wistar rats. CBF was measured by the intracarotid 133Xe injection method. In one study 1 and 10 mg/kg captopril respectively were given intravenously to SHR and WKY. This caused the blood pressure to fall transiently below the lower blood pressure limit of CBF autoregulation; nonetheless CBF remained normal during captopril induced hypotension. In a second study it was shown with an integral uptake method that captopril is a very poor penetrator of the blood-brain barrier. In a third study captopril was instilled intracerebroventricularly in Wistar rats without any effect on CBF or its autoregulation. Thus when given intravenously, but not intracerebroventricularly captopril caused cerebral vasodilatation during acute hypotension.