The arterial pressure level attained in sinoaortic denervated rats depends on the net effect of eliminating excitatory and inhibitory influences (chemoreceptor and baroreceptor elimination, respectively). After sinoaortic denervation is completed, the hypertension usually observed within the first few days is followed by normotension at the chronic stages. In this work renal sympathetic nerve activity was measured in conscious, unrestrained rats 6 hours (acute) and 20 days (chronic) after sinoaortic denervation. Increased arterial pressure (154±10 versus 114±3 mm Hg in controls) and renal sympathetic nerve activity (32±5 versus 13±2 bars per cycle in controls) with no changes in heart rate (404±17 vs 380±26 beats per minute) were observed in rats with acute sinoaortic denervation. In rats with chronic sinoaortic denervation, arterial pressure (119±8 mm Hg) and renal sympathetic nerve activity (13±6 bars per cycle) returned to control levels. Bradycardiac and tachycardiac responses to changes in blood pressure were reduced to 88% and 89%, respectively, in rats with acute sinoaortic denervation and 76% and 74%, respectively, in rats with chronic sinoaortic denervation. The reflex control of renal sympathetic nerve activity after acute and chronic sinoaortic denervation showed an impairment of sympathoinhibition (0.13±0.02 and 0.25±0.1 bars per cycle, respectively, versus 0.9±0.17 bars per cycle in controls). Sympathoexcitatory responses also were impaired in rats with acute and chronic sinoaortic denervation (0.08±0.03 and 0.37±0.1 bars per cycle, respectively, compared with 0.98±0.2 bars per cycle in controls). The variability of mean arterial pressure expressed by standard deviation was higher in rats with acute and chronic sinoaortic denervation (13±2 and 15±2 mm Hg, respectively) than in controls (6±3 mm Hg). The variability coefficient of renal sympathetic nerve activity was lower 6 hours (0.22±0.07) but normal 20 days after sinoaortic denervation (0.88±0.24 versus 0.77±0.06 in controls). We conclude that (1) acute sinoaortic denervation increases arterial pressure and renal sympathetic nerve activity and reduces renal sympathetic variability, (2) blood pressure and renal sympathetic activity return to normal levels in rats after chronic sinoaortic denervation, and (3) increased variability and impairment of the baroreflexes persisted in the chronic phase of sinoaortic denervation.