The authors have suggested from an in vitro study that extracellular K concentration can be elevated up to 40 Mm, when hemolysis of a clot occurs after subarachnoid hemorrhage. The present experiments were performed to investigate the change in contractility of rabbit basilar artery to vasoactive substances in the presence of increased K. The conclusion was as follows. 1. The contraction heights induced by serotonin (5 x 10(-7) M) were increased 2.4 +/- .02 fold, 6.4 +/- 0.3 fold, 5.7 +/- 0.4 fold (n = 10), when the K concentration of physiological solution was elevated to 8.1 mM, 13.5 mM, 24.3 mM, respectively. 2. Similar potentiation was obtained in prostaglandin F2 alpha. The tensions induced by prostaglandin F2 alpha (2.5 x 10(-6) M) were increased 2.3 +/- 0.3 fold, 4.7 +/- 0.4 fold, 3.7 +/- 0.4 fold (n = 6), whereas the responses to norepinephrine were rather less potentiated and the maximum height was 2.2 +/- 0.2 fold (n = 8) in 13.5 mM K. 3. Enhancement of the responses to vasoactive agents was also observed in the presence of tetraethylammonium (TEA). These results indicated that slight depolarization of membrane would potentiate the contractile responses to various vasoactive substances and that an increase in sensitivity could be a possible cause of cerebral vasospasm.