The calcium-sensitive neutral protease, calpain, is activated in the basilar artery after subarachnoid hemorrhage. Pathological activation of this proteolytic enzyme has been suggested to contribute to cerebral vasospasm after subarachnoid hemorrhage. The present study was undertaken to evaluate the effects of a newly developed calpain inhibitor, CX-287 on vasospasm. A blind, randomized trial was utilized in which CX-287 was injected intravenously into subarachnoid hemorrhage rabbits. Two days after subarachnoid hemorrhage, animals were sacrificed by perfusion-fixation and cross-sectional areas of the basilar arteries were measured using histological techniques. Expressing the cross-sectional area in the untreated SAH animals as a percentage of control value, it was 38.4+/-5.7%. Basilar artery area of the treatment groups with 1.5mg/kg CX-287(b.i.d. or t.i.d) showed no statistical differences from subarachnoid only group(b.i.d.: 34.7%, t.i.d.: 49.0%). However, the treatment group with 3mg/kg CX-287 showed significant reversal of the subarachnoid hemorrhage-induced constriction(b.i.d.: 73.4%, p<0.0003, t.i.d.: 58.7%, p<0.05). These findings support the important role of calcium activated proteolysis by calpain in the pathophysiology of vasospasm after subarachnoid hemorrhage. Furthermore, these results provide the first demonstration that a calpain inhibitor can inhibit cerebral vasospasm.