Cerebral blood flow (CBF) was measured and cerebrovascular resistance (CVR) was calculated in anesthetized spontaneously hypertensive rats (SHR) and normotensive rats (NR) following the administration of incremental dosages of i.v. flunarizine or papaverine. CBF and CVR changes following papaverine were the same in both groups of rats irrespective of the dose of the drug. The effect of flunarizine was much more pronounced in SHR than in NR. The results point out the greater dependency of basal cerebrovascular tone in SHR upon Ca2+ influx into vascular smooth muscle cells.