&NA;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.