Untrained, moderately trained (runners, 15 to 25 mi/wk), and highly trained (runners, greater than 45 mi/wk) men participated in graded treadmill exercise at 50%, 70%, and 90% of their maximal oxygen consumption to quantify the relation between intensity of exercise and sympathetic nervous system and metabolic responses. Sympathetic system activation was noted at all intensities tested and was proportional to the relative exercise intensity. The magnitudes of the norepinephrine (NE) and epinephrine (E) responses were similar in all three groups of men at each relative exercise intensity and correlated with the magnitudes of change in levels of circulating plasma adrenocorticotropin hormone, cortisol, lactate (La), phosphate (Pi), and glucose (GI). The magnitudes of change in concentrations of La, Pi, and GI were also similar for the three groups at each relative exercise intensity. In contrast, a lower degree of sympathetic system activation in response to a given absolute workload was noted in the moderately and highly trained men as compared to that of the untrained men. Sympathetic and metabolic responses to exercise are similar under conditions of comparable relative exercise intensities, regardless of conditioning level. The sympathetic-adrenal medullary system is more sensitive to exercise than the hypothalamic-pituitary-adrenal axis. For a given absolute workload, the degree of activation significantly lower in trained individuals.