The characteristics of rapid or acute resetting of the aortic baroreceptors were studied in anesthetized rats during 30 minutes of sustained hypertension produced by phenylephrine infusion. Two minutes after the onset of hypertension, a resetting of 26% (ratio of change in systolic threshold pressure to total change in diastolic pressure) was demonstrable. This represents 60% of the maximal resetting (43%) observed after 20 minutes. Thereafter, the magnitude of resetting remained stable. Pressure-nerve activity showed a parallel shift after 2 and 20 minutes of hypertension, with a slight tendency to increased slope at elevated pressures, especially in the baroreceptor function curve analyzed after 2 minutes. These results suggest that in rats, no significant difference exists in the time course for resetting of baroreceptor fibers with low vs. high thresholds. In contrast to other studies, reversibility of the resetting process was not complete within 30 minutes of pressure normalization, and the extent of reversal was no greater than 60%. The data of the present study, taken together with those obtained previously after 6 hours of hypertension, suggest that during the onset and maintenance of hypertension in rat, acute or rapid resetting of the baroreceptors reaches its maximum in 20 minutes (40%) and remains stable for up to 6 hours, with no apparent change in the baroreceptor gain.