The presence of two types of carotid sinus baroreceptors, as characterized by two different stimulus-response curves in an earlier study, suggests that each type may play a different role in the regulation of blood pressure. The discontinuous hyperbolic curve of the type I baroreceptors, marked by higher firing rates and greater sensitivity than the sigmoidal curve of type II baroreceptors, suggests that these baroreceptors would contribute more to the buffering of arterial pressure changes than the “tonically” active type II baroreceptors, which fired over greater pressure ranges and generally had spontaneous subthreshold discharge. The firing characteristics of type II baroreceptors suggest that these receptors would contribute more to regulation of tonic, baseline levels of arterial pressure. If this functional differentiation exists, the acute resetting characteristics of the two types of baroreceptors could be different. Resetting is defined as a shift in the response curve of a baroreceptor, marked by shifts in pressure threshold, in the same direction as the change in pressure to which it is exposed. Type I baroreceptors would be more likely to reset in response to a sustained acute change in pressure, since their primary role would be to prevent the initial change in pressure. However, type II baroreceptors would not reset to the acute change in pressure, since their primary role would be to maintain consistent information on the level of existing pressure. Therefore, this study was performed to examine the acute resetting ability of both types of baroreceptors by using a vascularly isolated carotid sinus preparation in the dog. Single-fiber type I and II baroreceptor recordings were obtained during slow ramp changes in sinus pressure after pulsatile conditioning at mean pressures of 50, 100, and 150 mm Hg for 15–25 minutes. Seven type I baroreceptors were also conditioned using nonpulsatile mean pressures of the same values. Firing characteristics, including pressure threshold, were determined using nonlinear regression and best-fit analysis. Type I, but not type II, baroreceptors were found to reset to acute changes in pressures. There was no significant difference in resetting of type I baroreceptors in response to pulsatile versus nonpulsatile conditioning pressures. These results indicate that the resetting abilities of the two types of baroreceptors are different and suggest the possibility of functionally different roles for each in the regulation of the cardiovascular system. (Circulation Research1992;70:559–565)