We have recently demonstrated that K(+)-induced dilation of cerebral resistance-sized vessels has two independent components, only one of which seemed sodium pump dependent. In our current investigation, potassium-induced dilation of spontaneous tone was compared in cerebral arteries from normotensive Wistar-Kyoto rats and age-matched stroke-prone spontaneously hypertensive rats. Branches of the posterior cerebral artery were cannulated and pressurized, and these vessels developed spontaneous tone. After a 5-minute period in K(+)-free physiological saline solution, K+ was increased in 1-mM increments to a final concentration of 15 mM. In the normotensive arteries, K+ concentrations between 0 and 5 mM K+ resulted in dilations that had a transient (sodium pump-dependent) component, and K+ concentrations in excess of 7 mM produced dilations that lacked a transient (sodium pump-independent) component. Similar branches from the hypertensive rat also responded with transient dilations to K+ (less than 5 mM), and these were significantly greater at 3 mM K+. However, the maintained dilations to K+ (greater than 7 mM), noted in preparations from Wistar-Kyoto rats, were absent in seven of eight preparations. Thus, the impaired dilations, in the hypertensive vessels, to K+ described here is a consequence of altered function of some sodium pump-independent component rather than altered Na+,K(+)-ATPase activity.