Lidoflazine, a piperazine derivative of known selectivity for vascular smooth muscle, was evaluated as a possible agent for prophylaxis of cerebral vascular contraction induced by subarachnoid perfusion with whole blood. Previous studies from this laboratory have indicated its efficacy in preventing basilar artery contraction induced by serotonin. The animals treated with a subarachnoid perfusion of whole blood had a mean 30% reduction in vessel diameter over the control value. Groups that were treated with 0.5 mg/kg of lidoflazine and 1.0 mg/kg of lidoflazine and then perfused with whole blood in the subarachnoid space had reductions in control diameter of 2.8% and 6.8%, respectively. One group treated with 2.0 mg/kg of lidoflazine and then perfused with whole blood actually had an increase in diameter of 6.8% over the control value. Lidoflazine, when administered intravenously at a slow rate, will not adversely lower systemic blood pressure and can prevent the contraction of cerebral vessels when the stimulus for contraction is whole blood within the subarachnoid space.