Twelve patients with essential hypertension (WHO stages I-II) were subjected to mild aerobic exercise for 10 to 20 weeks. The time course of changes in the resting blood pressure and multiple hormonal responses (plasma catecholamines, prostaglandin E, renin-angiotensin system, kallikrein-bradykinin system) were monitored. Depressor response of both systolic and diastolic pressures was seen, and after 5 weeks of exercise blood pressure stabilized at a significantly lower level. Adjustment of work load in response to increased physical fitness at the 10th week produced further reduction of blood pressure especially in diastole. After exercise therapy we found significant reductions in plasma catecholamine levels, and increases in levels of plasma prostaglandin E and the urinary excretion of sodium. A reduction in systolic/diastolic (mean) pressures by more than 20/10 (13) mm Hg was seen in 50% of patients after 10 weeks and in 78% after 20 weeks of exercise. Those who achieved effective blood pressure fall after 10 weeks of training (n = 6) were compared with the rest (n = 6). This analysis revealed that the initial value of plasma renin activity of the former was significantly lower than that of the latter. Significant negative correlations (r = -0.78) also were observed between the blood pressure reduction and corresponding initial value of plasma renin activity. These results indicate that exercise therapy is a potent nonpharmacological tool for the treatment of essential hypertension, especially of the low renin type. Both diminished sympathoadrenergic activity and enhancement of prostaglandin mechanisms might be responsible for the falls in arterial pressure.