The sympathetic nervous system controls cardiovascular responses during postural changes and exercise. In addition, norepinephrine and epinephrine have effects on metabolism and hormone secretion that are important durн ing exercise. Norepinephrine, epinephrine and dopamine also function as neurotransmitters within the central nervous system. This review is conн cerned specifically with sympathetic nervous activity during exercise. After a brief summary of the methods of determining sympathetic nervous activн ity, we discuss its significance in cardiovascular control, fuel mobilization, and hormone secretion during exercise. We next consider some physiologiн cal factors that modify sympathetic nervous activity during exercise. Fiн nally, we emphasize the importance of regulatory mechanisms by noting the changes in sympathetic nervous activity seen in a few pathophysiological conditions.