Vascular endothelium synthetizes mediators which either cause vasorelaxation or vasoconstriction. The purpose of this study is to investigate the role of these mediators in the pathogenesis of cerebral vasospasm. An inhibitor of endothelium-derived relaxing factor is haemoglobin; endothelium removal by triton X-100 and endothelin-1 caused narrowing in the diameter of basilar artery of the rabbit. The pathological, ultrastructural investigations and computerized tomography images are in accordance with this finding. These results indicate that not only an increase in vasoconstrictor mediator level but also the lack of endothelium-derived relaxing mediators are responsible of the spasm of the basilar artery.