This study attempted to measure the extent to which the carotid sinus baroreceptor reflex attenuates the acute changes in arterial pressure during infusions of phenylephrine and nitroprusside, and to express these effects as equations which could be used in a simple computer model of the circulation. Arterial pressure and cardiac output were measured in vagotomized cats anesthetized with chloralose or pentobarbital. The carotid arteries were perfused to allow sinus pressure either to follow changes in systemic arterial pressure or to be held at the preinfusion control level, thereby eliminating the effects of the reflex. There was a linear relationship between arterial pressure in the presence and absence of the reflex, and the changes in arterial pressure were attenuated to one-third by the reflex while cardiac output was not significantly changed. These effects are described by simple equations involving peripheral resistance, heart rate, and venous compliance. The activity of the reflex was not significantly different in cats anesthetized with pentobarbital or chloralose, and our data do not support the view that these anesthetic agents depress the vasomotor component of the carotid sinus reflex.