Cerebral vasospasm associated with aneurysmal subarachnoid hemorrhage (SAH) remains a major complication in patients suffering from SAH. Regulation of membrane potential of arterial smooth muscle through activation or inhibition of potassium (K+) channel activity provides an important mechanism to dilate or constrict arteries. The present study examined the effect of a K+ channel activator, cromakalim, on cerebral vasospasm following experimental SAH. By the route of topical application and intra-arterial injection, basilar arteries were exposed transclivally and measured on-line using videomicroscopic camera. Continuous microinjection from right vertebral artery was given after the result of application was observed. Basilar artery spasm induced by SAH was released by topical or intra-arterial administration of cromakalim, and this beneficial effect against cerebral vasospasm was dose-dependent. There was no significant difference between topical and intra-arterial administration of cromakalim. These results indicate that K+ channel activator may play an important role for ameliorating cerebral vasospasm. An important goal of future studies will be to carefully evaluate the possibility and effect of intra-arterial administration of cromakalim to treat angiographic vasospasm.