&NA;Hemolysis of periarterial clots after subarachnoid hemorrhage may liberate large quantities of K+ into the vicinity of cerebral blood vessels and possibly change their sensitivity to endogenous vasoactive agents. The current study examined the influence of subarachnoid hemorrhage on the sensitivity of rabbit basilar arterial segments to K+ and Ca++. An analysis of K+ and Ca++ dose‐response curves demonstrated that incubated arterial segments isolated from animals with subarachnoid hemorrhage were substantially more sensitive to these cations than were corresponding controls. We speculate that chronically elevated K+ levels in areas of periarterial clot lysis or brain ischemia might initiate vascular smooth muscle depolarization and vasospasm. Our data provide additional rationale for the use of calcium channel blockers in preventing or treating vasospasm in cases of subarachnoid hemorrhage. (Neurosurgery19:346‐349, 1986)