The present study examined the time course of changes in baroreceptor reflex function by means of sigmoidal curvefitting analysis in conscious, unrestrained renovascular one-kidney, one clip (1K1C) rats at 1, 3, 7, 15, 30, and 60 days after renal artery clipping. The reflex heart rate responses were elicited by alternate intravenous bolus injections of phenylephrine (change, +5 to +50 mm Hg) and sodium nitroprusside (change, −5 to −50 mm Hg). Atropine methylnitrate and atenolol were given to evaluate the responses mediated by the cardiac sympathetic or vagal component, respectively. The average baroreceptor reflex gain (sensitivity) decreased progressively (day 1, 3.35±0.3 beats per minute [bpm] per millimeter of mercury), reaching a maximal attenuation in the 30-day 1K1C group (1.83±0.5 bpm/mm Hg) compared with sham rats (approximately 4.60 bpm/mm Hg). The data showed a decreased vagal activity contributing to the attenuation of the baroreceptor gain only in the 30-day 1K1C group. In contrast, the cardiac sympathetic component of the baroreceptor reflex was significantly decreased in all 1K1C groups (from 2.10±0.4 to 0.50±0.2 bpm/mm Hg) compared with the respective sham groups (from 3.80±0.3 to 3.10±0.4 bpm/mm Hg). These results suggest that a reduced contribution of the sympathetic component to the baroreceptor heart rate reflex may be the main cause of the progressive attenuation of the baroreceptor reflex sensitivity observed in conscious 1K1C hypertensive rats.