We studied: (1) the effects of an abrupt, moderate increase in arterial pressure on total and regional cerebral blood flow (CBF) and (2) whether sympathetic stimulation attenuates the transient hyperemia that occurs during a sudden increase in pressure within the physiological range of pressure. Abrupt increases in arterial pressure were produced by occlusion of the descending aorta. Cerebral blood flow was determined in dogs and cats using radioactive microspheres. In dogs, blood flow to all regions of the brain increased by 35-55% at the onset of hypertension and returned to normal by 60 seconds. Electrical stimulation of sympathetic nerves did not attenuate the transient rise in CBF in dogs. In cats, blood flow increased by 40-60% in cerebrum (cortical grey matter), cerebellum, and brainstem at the onset of hypertension and was still moderately elevated after 2.5 minutes. Electrical stimulation of sympathetic nerves in cats attenuated the initial rise in CBF. At the onset of hypertension in cats, the increase in blood flow to the unstimulated cerebrum was 42% greater than on the stimulated side. Blood flow to cortical grey matter was 71% and 65% greater on the unstimulated side than on the stimulated side at the onset and after 20 seconds of hypertension, respectively. We conclude that an abrupt, moderate increase in arterial pressure within the physiological range produces a transient increase in CBF and, furthermore, that stimulation of sympathetic nerves attenuates the increase in flow in cats. Circ Res 46: 696-702, 1980