The effects of hemoglobin and cerebrospinal fluid from patients with subarachnoid hemorrhage (CSF-SAH) on endothelium-dependent relaxation were studied. At 10(-6) M, hemoglobin somewhat inhibited the endothelium-dependent relaxation induced by A23187 in rings of canine basilar artery. At 3 X 10(-6) M, it almost completely inhibited the same response. At 3 X 10(-6) M, hemoglobin did not significantly inhibit smooth muscle relaxation mechanisms as papaverine-induced relaxation was not inhibited by hemoglobin. It was also demonstrated that pretreatment of arterial rings with CSF-SAH resulted in a dose-dependent inhibition of relaxation induced by A23187. The inhibitory effect of CSF-SAH was prominent in the case in which a high oxyhemoglobin concentration was measured by spectrophotometry. Normal CSF from patients without SAH did not affect endothelium-dependent relaxation. These results suggest that hemoglobin released from lysed erythrocytes inhibits endothelium-dependent relaxation of canine basilar arteries and may also play an important role in the pathogenesis of cerebral vasospasm after aneurysmal subarachnoid hemorrhage.