Muscle sympathetic activity in the peroneal nerve and ECG were measured during pharmacologic manipulations of blood pressure in eight healthy adults. The frequency characteristics of an integrated nerve signal and a derived heart-rate signal were determined and compared to two traditional measures of sympathetic activity--burst count/minute and plasma norepinephrine. It was found that modulation of the nerve activity occurs over a broad range from 0. 03 Hz up to the heart rate, and that muscle sympathetic activity may or may not correlate with low frequency heart rate fluctuations, depending on the parasympathetic input to the sinoatrial node. A model for the observed behavior is proposed.