This study was undertaken to examine the effect of increasing transmural pressure on membrane electrical properties of cat middle cerebral arterial muscle. Middle cerebral arteries were removed from the cat brain, cannulated, and prepared so that transmural pressure within a segment could be manipulated. Intracellular membrane potential was recorded with glass microelectrodes at various transmural pressures. There was a positive slope relating changes in intracellular membrane potential as a function of transmural pressure with a correlation coefficient of 0.79. Blockade of nerve excitation with tetrodotoxin and inhibition of alpha-adrenergic receptors with phentolamine not only did not block the pressure-induced depolarization, but increased the slope of the intracellular membrane potential vs. pressure relationship. This slope was increased upon elevation of extracellular calcium concentration from 2.5 to 4.0 mM and was significantly reduced upon reduction of extracellular calcium concentration to 0.5 mM. When arterial preparations were equilibrated at 0 mm Hg prior to pressurization, action potentials were recorded only when pressure was initially elevated, while a sustained depolarization was recorded during the pressure plateau. However, when arteries were equilibrated at a transmural pressure of 100 mm Hg for 90 minutes, spontaneous action potentials were recorded which increased in frequency as a function of pressure until they were inactivated when intracellular membrane potential approached -30 mV at high transmural pressures. Photomicrographs demonstrated that these vessels either maintained or decreased diameter upon pressurization. These findings provide a cellular mechanism for myogenic regulation of cerebral arterial diameter.