Cerebral vasospasm is the most severe complication of subarachnoid hemorrhage. Although the etiology of vasospasm is still obscure, it has been suggested that some Ca antagonists can relieve cerebral vasospasm. The effects of Ca antagonists (Nifedipine, Nicardipine, Verapamil, Cinnarizine, Diltiazem) on the diameter of the basilar artery and rCBF in the brain stem were investigated in cats with experimental cerebral vasospasm. Three days after an intracisternal injection of blood (3ml), the basilar artery was exposed by the transclival approach. Experimental cerebral vasospasm was induced by the topical application of a blood CSF mixture. The rCBF was measured by the heat clearance method, and the diameter of the basilar artery was examined in serial photographs. Whenever Ca antagonists were given, the mean arterial blood pressure (MABP) decreased dose dependently. Ca antagonists can dilate (from 7.1% to 45% 10 minutes after the administration of Ca antagonists) the normal basilar artery, but can hardly change the basilar artery of cats with experimental cerebral vasospasm. The response of the rCBF in the brain stem after the administration of Ca antagonists was of 3 types: continuous increase, transient increase and no response. The greater the MABP decreased, the higher the possibility of continuous increase and transient increase among all cases was. The possibility of continuous increase and that of transient increase were higher in the control group than that in the vasospasm group. However, even in the vasospasm group the possibility of continuous rCBF increase and that of transient rCBF increase in the brain stem were not above 50%. Therefore, the clinical application of Ca antagonists to the treatment of vasospasm after subarachnoid hemorrhage may be difficult. But, on the other hand, it is possible that the administration of Ca antagonists before the appearance of vasospasm would be the prophlaxis for vasospasm after subarachnoid hemorrhage.