The effects of a 16-week aerobic exercise program on blood pressure and plasma catecholamine levels were evaluated in 56 patients with baseline diastolic blood pressure of 90 to 140 mm Hg. The exercise group significantly improved their physical fitness, and reduced systolic and diastolic blood pressures, compared with controls. To evaluate the relationship between exercise, blood pressure, and plasma catecholamine values, the exercise group was further divided into hyperadrenergic and normoadrenergic subgroups. Reductions in systolic pressures were 6.3 mm Hg, 10.3 mm Hg, and 15.5 mm Hg for control, normoadrenergic, and hyperadrenergic groups, respectively. Diastolic changes were similar and also significant. Within the hyperadrenergic group, changes in blood pressures were associated with changes in values for plasma catecholamines following training. We conclude that an aerobic exercise program reduces blood pressure, which is at least partially mediated by changes in plasma catecholamine levels.