Certain electrophysiological properties of smooth muscle cells within middle cerebral arteries of spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto (WKY) controls were recorded with glass microelectrodes. Under control conditions the smooth muscle cells of arteries from SHR rats exhibited fast and slow oscillations of the resting membrane potential (E<sub>m</sub>). In marked contrast the smooth muscle cells of WKY cerebral arteries were electrically quiescent. The E<sub>m</sub> vs. log [K]o relationship was similar for arteries from both SHR and WKY rats and extrapolated to similar [K]i values of 150–155 mM when determined under control conditions. When E<sub>m</sub> vs. log [K]o curves were obtained in the presence of ouabain to block electrogenic Na-K transport, the smooth muscle cells of SHR cerebral arteries exhibited a lower mean slope per decade change in [K]o (i.e., depolarized more at various concentrations of [Ko in the presence of ouabain) when compared to WKY. Arterial smooth muscle cells from SHR also depolarized to a greater extent when exposed to zero K<sup>+</sup> solutions. The results of this study indicate that smooth muscle cells of cerebral arteries from SHR rats have a greater electrogenic E<sub>m</sub> component as well as altered ionic conductances for K<sup>+</sup> and possibly Na<sup>+</sup> both of which may contribute to their spontaneous electrical activity.