The present study examined whether the gain of the transfer function relating the cardiac-related rhythm of renal sympathetic nerve activity (RSNA) to the arterial pressure (AP) pulse might serve as a spontaneous index of sympathetic baroreflex sensitivity. AP and RSNA were simultaneously recorded in conscious rats, either baroreceptor-intact (control, n=11) or with partial denervation of baroreflex afferents (aortic baroreceptor denervated, ABD) (n=10) during 1-h periods of spontaneous activity. The transfer gain was calculated over 58 adjacent 61.4-s periods (segmented into 10.2-s periods). Coherence between AP and RSNA was statistically (P<0.05) significant in 90±3% and 56±10% of cases in control and ABD rats, respectively. The transfer gain was higher (P=0.0049) in control (2.39±0.13 normalized units (NU)/mmHg) than in ABD (1.48±0.22 NU/mmHg) rats. In the pooled study sample, the transfer gain correlated with the sympathetic baroreflex sensitivity estimated by the vasoactive drug injection technique (R=0.75; P<0.0001) and was inversely related to both time- (standard deviation; R=-0.74; P=0.0001) and frequency-domain (total spectral power (0.00028-2.5 Hz); R=-0.82; P<0.0001) indices of AP variability. In control rats, the transfer gain exhibited large fluctuations (coefficient of variation: 34±3%) that were not consistently related to changes in the mean level of AP, heart rate or RSNA. In conclusion, the transfer function method provides a continuous, functionally relevant index of sympathetic baroreflex sensitivity and reveals that the latter fluctuates widely over time.