Vasospasm of the basilar artery of the dog was induced by subarachnoid injection of arterial blood through the cisterna magna. Cerebral angiography was employed to evaluate quantitative assessment of the spasm. Chronic vasospasm was successfully induced in 100% of surviving dogs.Biphasic vasoconstriction was observed. The acute phase occurred within 30 minutes after the blood injection and tended to abate. Chronic spasm was demonstrated on the second day's angiograms and persisted to the seventh day in some cases. Etiology of the chronic spasm using this model is now under investigation.Vasospasm was not due to alterations in blood gas concentration or blood pressure, to increased CSF pressure or to injury from contrast medium or direct trauma. The primacy of blood as the offending agent is strongly suggested.