Vasospasm of the basilar artery of the cat was induced by subarachnoid bleeding and was observed by a transoral approach to the vessel. Intravenous Isordil (2 to 4 mg/kg) given following vasospasm produced vasodilatation with minimal arterial hypotension. Cyclospasmol, administered intravenously after vasospasm or orally prior to vasospasm, was ineffective in relieving or preventing basilar artery spasm.