Effects of a new synthetic vasodilator, YC-93, a derivative of 1. 4-dihydropyridine, on experimental cerebral vasospasm, were studies in cats. A topical administration of YC-93 solution was found to rapidly reverse the spasm of the basilar artery induced with a subarachnoi application of fresh blood, serotonin or porstaglandin F2 alpha. Pretreatment of the basilar artery with YC-93 prevented the development of experimental vasospasm with all these agents.