Canine cerebral arterial segments tested in a tissue bath were found to escape from the vasoconstrictor effects of serotonin, the basilar segments more rapidly than the middle cerebral segments. At low doses (10−9M), serotonin caused a small, prolonged contraction, whereas at higher doses (10−6M), a more forceful phasic response was seen. Verapamil blocked the phasic portion of the response but not the low amplitude sustained portion. Increasing the bath concentration of K+, Ba++, and tetraethylammonium markedly enhanced the constrictor response of low serotonin doses and significantly inhibited escape. Potentiation of the force and duration of vasoactive amine-induced arterial constriction by small increments of extracellular K+ might be important in the prolonged vascular narrowing associated with subarachnoid hemorrhage.