These studies were designed to determine the role of cerebral vascular endothelium in the “myogenic” depolarization and contraction observed in isolated cat middle cerebral arteries exposed to high transmural pressures. With intact endothelial cells we observed, on elevation of transmural pressure in cannulated isolated arteries, significant membrane depolarization, action potential generation, and reduction in internal diameter. After perfusion of the same vessels with collagenase and elastase for short periods of time to disrupt the endothelial layer, all previous responses to elevation of transmural pressure were no longer seen. Even though enzyme perfusion had no effect on membrane potential at “control” levels of transmural pressure, it abolished the pressure-dependent depolarization, action potential generation, and constriction. Furthermore, the contractile response to agonist stimulation was maintained after endothelial disruption via enzymes, showing that this method of endothelial disruption did not appreciably damage muscle cells. The data document a dependence of an intact endothelium in mediating the activation of isolated cat cerebral arteries in response to a changing transmural pressure. Thus, it is possible that the endothelial cell may serve as a transducer in the autoregulatory response to pressure.