To investigate whether the rate of change in blood pressure affects the sensitivity of the aortic baroreceptor afferent response, the change in aortic nerve activity (ANA) to two different rates of ramp increase in mean blood pressure (MBP), elicited by phenylephrine administration, was determined in the rat under urethane (1.5 g kg(-1)) anesthesia. The sensitivity of the increase in ANA following a rapid (average ramp rate, 9.14+/-0.60 mm Hg s(-1), n=11) or gradual (1.78+/-0.24 mm Hg s(-1), n=11) increase in MBP was 2.03+/-0.14% and 1.81+/-0.20% of baseline mm Hg(-1), respectively. These values were not significantly different from each other (P=0.16). Furthermore, we found no correlation between the rate of ramp increase in MBP and the sensitivity of the increase in ANA (r=0.24, P=0.29, n=22). These results suggest that, at least within the normal physiological range of MBP, the rate of the ramp change in blood pressure does not affect aortic baroreceptor afferent sensitivity in the anesthetized rat.