[K⁢]O can be increased under a variety of conditions including subarachnoid hemorrhage. The increase of [K⁢]O in the range of 5∼15 mM may affect tensions of blood vessels and can change their sensitivity to various vasoactive substances. Therefore, it was examined in the present study whether the sensitivity of cerebral arteries to vasoactive substances can be changed with the moderate increase of [K⁢]O, using Mulvany-type myograph and [Ca2⁢]c measurement. The contractions of basilar artery and branch of middle cerebral artery induced by histamine were not increased with the elevation of [K⁢]O from 6 mM to 9 mM or 12 mM. On the contrary, the contractions induced by serotonin were significantly increased with the elevation of [K⁢]O. The contractions were also significantly increased by the treatment with nitro-L-arginine (10⁣4 M for 20 minutes). In the nitro-L-arginine treated arteries, the contractions induced by serotonin were significantly increased with the elevation of [K⁢]O from 6 mM to 12 mM. K⁢-induced relaxation was evoked with the stepwise increment of extracellular K⁢ from 0 or 2 mM to 12 mM by 2 mM in basilar arterial rings, which were contracted by histamine. But [K⁢]O elevation from 4 or 6 mM to 12 mM by the stepwise increment evoked no significant relaxation. Basal tension of basilar artery was increased with [K⁢]O elevation from 6 mM to 12 mM by 2 mM steps or by the treatment with ouabain and the increase of basal tension was blocked by verapamil. The cytosolic free Ca2⁢ level was not increased by the single treatment with serotonin or with the elevation of [K⁢]O from 4 mM to 8 or 12 mM. In contrast to the single treatment, the Ca2⁢ level was increased by the combined treatment with serotonin and the elevation of [K⁢]O. The increase of free Ca2⁢ concentration was blocked by the treatment with verapamil. These data suggest that the sensitivity of cerebral artery to serotonin is increased with the moderate increase of [K⁢]O and the increased sensitivity to serotonin is due to the increased [Ca2⁢]i induced by extracellular Ca2⁢ influx.