Plasma renin activity (PRA), renin concentration (PRC), angiotensin II and urinary aldosterone of four male athletes were investigated before and after a running exercise of 3 X 300 m. After the exercise, there were marked increases in all these parameters. The maximal increases (of the means and the ranges), found in the samples taken 30 min after the exercise, were: 108% (27-230%, P less than 0.05) in PRA, 490% (240-800%, P less than 0.01) in PRC, 830% (400-1,970%, P less than 0.025) in plasma angiotensin II and 1,600% (160-3,920%, P less than 0.02) in plasma aldosterone. The increase in the urinary excretion of aldosterone was 120% (42-180%, P less than 0.025). This study demonstrates that intense physical exercise may cause marked changes in all the three main components of the renin-angiotensin-aldosterone system. The significance of these changes for the physiological function of the human organism in physical stress needs further investigation.