The cerebrovascular effects of the converting enzyme inhibitor captopril were examined after intravenous or intra-cerebroventricular administration in nornotensive and hypertensive rats. Cerebral blood flow (CBF) was measured using the intracarotid 133xenom method in halothane/nitrous oxide anaesthetised animals. Captopril given either way did not influence the absolute value of CBF. The main finding, however, was that following intravenous administration of captopril at a dose af 10 mg/kg, both the lower pressure limit of CBF autoregulation were shifted to lower pressure and the autoregulatory plateau shortened. The lower limit was shifted 20-30 mm Hg, the upper limit 50-60 mm Hg, andthr autoregulatory plateau consequently shortened by 20-30 mm Hg. In contrast to the marked effects of intrvenous captopril, intracerebroventricular captopril was without effect on CBF autoreguletion. The effect of intravenous captopril was thus probably mediatedby converting enzyme in the cerebrovascular endothelium.