The effect of physical endurance training on the plasma renin-angiotensin-aldosterone system was studied in 27 normal sedentary volunteers aged between 20 and 55 years, using a randomized two-period cross-over study design. After 4 months of training (2.5 h/week), peak oxygen uptake and physical working capacity at a heart rate of 130 beats/min were increased by 16% (P less than 0.01) and 29% (P less than 0.001) respectively, whereas resting heart rate was decreased by 15% (P less than 0.001). The plasma noradrenaline concentration and haematocrit were both decreased (P less than 0.01) after training. For the total group of subjects, the small decreases in plasma renin activity (PRA) and in the plasma concentrations of angiotensin-I, angiotensin-II and aldosterone were not statistically significant. However, the change in PRA during the training period was negatively correlated with the increase in physical working capacity (r = -0.49, P less than 0.01), suggesting that PRA decreased only in those subjects with the greatest increase in exercise capacity. Also, the change in plasma aldosterone during training was negatively related to the rise in physical working capacity (r = -0.57, P less than 0.01). Furthermore, the changes in plasma angiotensin-I (r = 0.75), angiotensin-II (r = 0.49) and aldosterone (r = 0.43) during the training period correlated positively with the change in PRA. It is concluded that physical endurance training, leading to a substantial gain of physical working capacity, suppresses the plasma renin-angiotensin-aldosterone system in normal man.