Muscle sympathetic nerve activity (MSNA) in resting humans is characterized by cardiacrelated bursts of variable amplitude that occur sporadically or in clusters. The current study was designed to characterize the fluctuations in the number of MSNA bursts, interburst interval, and burst amplitude recorded from the peroneal nerve of 15 awake, healthy human subjects. For this purpose, we used Allan and Fano factor analysis and dispersional analysis (DA) to test whether the fluctuations were time-scale invariant (i.e., fractal) or random in occurrence. Specifically, we measured the slopes of the power laws in the Allan factor, Fano factor, and DA curves. In addition, the Hurst exponent was calculated from the slope of the power law in the Allan factor curve. A decision as to whether the original time series contained fractal fluctuations was based on a comparison of the values of these parameters with those for surrogate data blocks. The results can be summarized as follows. Fluctuations in the number of MSNA bursts and interburst interval were fractal in each of the subjects, and those in burst amplitude were fractal in four of the subjects. We also found that fluctuations in the number of heart beats and heart period (R-R interval) were fractal in each of the subjects. These results demonstrate for the first time that apparently random fluctuations in human MSNA are, in fact, dictated by a time-scale invariant process that imparts "long-term memory" to the sequence of cardiac-related bursts. Whether sympathetic outflow to the heart also is fractal and contributes to the fractal component of heart rate variability remains an open question.