The plasma concentrations of aldosterone and its known regulators, plasma renin, potassium and ACTH, were examined during graded intensities of treadmill exercise (50, 70 and 90% of maximal oxygen uptake, VO2max). Sedentary men (n = 7) and two groups of runners of different training status (moderately trained, 15-25 miles/week, n = 7; highly trained, greater than 45 miles/week, n = 7) were studied in an attempt to define whether physical training causes changes in aldosterone homeostasis. Acute exercise was associated with elevations in plasma aldosterone, renin activity, potassium and ACTH in all three groups of subjects at exercise intensities of 70 and 90% VO2max. There were no differences in any of the responses among the three groups except for a blunted response of PRA at 90% VO2max in highly trained athletes. The exercise-induced rise of plasma aldosterone concentration did not correlate with changes in the concentration of its regulatory substances. We conclude that exercise stimulates the renin-angiotensin-aldosterone axis in an intensity-dependent fashion. With increased physical training identical hormonal and metabolic responses result at increased absolute workloads.