Abstract A quantitative relationship between power densities of blood pressure ( P BP) and sympathetic nerve activity ( P SNA) in a low-frequency range (LF, 0.016–0.85 Hz), expressed as P SNA = P BP × a × 10 b×(frequency) was proposed in pentobarbital-anesthetized rats. For evaluating the general applicability of this equation, the quantitative relationship of power density ratio H( f) = P BP/ P SNA across frequency was tested in a conscious state. Wistar rats were chronically instrumented with a femoral artery catheter and recording electrode around the renal sympathetic nerve. The blood pressure and renal sympathetic nerve activity were monitored both under pentobarbital anesthesia and in a conscious state. Linear regression analysis of the relationship between the frequency and logarithmic magnitude of the power density ratio in the LF range revealed excellent fit in both conditions ( r = − 0.96 ± 0.01 and − 0.93 ± 0.01 for anesthetized and conscious rats, respectively). Comparing the regression lines, rats under pentobarbital anesthesia had significantly larger values for the y-intercept and slope compared to rats in a conscious state ( y-intercepts: 0.80 ± 0.09 > 0.53 ± 0.08; slopes: − 2.86 ± 0.26 > − 1.62 ± 0.21). Our results demonstrate that it is also feasible to use the weighted P BP in LF as a quantitative index of sympathetic variability in conscious rats, but the evaluation of possible complications controlling the regression parameters is called for.