The hemodynamic alterations produced by large cerebral arteriovenous aneurysms are qualitatively similar to those caused by arteriovenousaneurysms elsewhere in the body. Congestive heart failure was not observed in our eight patients; it appears to be unusual in adult cases.By comparing cardiac outputs with a control group the mean aneurysm blood flow in six patients was estimated to be 2,804 ml. per minute at rest. Isoproterenol increased cardiac output relatively more than the estimated aneurysm flow.Implications of the present data with respect to the concept of “central” blood volume are discussed.Progressive mental and neurologic deterioration is the rule with large cerebral arteriovenous aneurysms. Repeated bleeding episodes, which may not be obvious clinically, appear to be partially responsible.