The purpose of this study was to determine directly segmental cerebral vascular resistance during sympathetic stimulation and changes in arterial pressure. We measured pressure in pial arteries in anesthetized rabbits and cats with a servo-null pressure-measuring device. Cerebral blood flow was measured with microspheres. Using these measurements we calculated large artery resistance and small vessel resistance. Under control conditions, large artery resistance accounted for approximately 40% of total cerebral vascular resistance. Sympathetic stimulation increased large artery resistance and reduced pial artery pressure. Cerebral blood flow and total cerebral vascular resistance did not change significantly. To examine constrictor responses of small cerebral vessels, we raised cerebral perfusion pressure by obstructing the descending aorta. During increases in arterial pressure from 70 to 110 mm Hg, large artery resistance tended to increase and small vessel resistance increased significantly. We conclude that, although sympathetic stimulation has little effect on total cerebral vascular resistance under normal conditions, it has important effects on segmental vascular resistance and cerebral microvascular pressure, and that sympathetic stimulation and increases in systemic arterial pressure within the physiological range have markedly different effects on segmental resistance; i.e., sympathetic stimulation produces constriction only in large arteries, and increases in systemic arterial pressure within the physiological range produce constriction primarily in small vessels.