Plasma renin activity (PRA) and aldosterone concentration were measured before and during submaximal exercise in 10 male monozygotic twin pairs who were discordant for smoking. In nine twin pairs PRA was higher in the smoker both at rest and during exercise. The mean PRA was 99% higher at rest and 84% higher during exercise than in nonsmokers. Plasma aldosterone levels were higher at rest in seven smokers and during exercise in eight smokers compared with the respective nonsmokers. The mean aldosterone level at rest was 23% and during exercise 40% higher in the smokers than in the nonsmokers. Chronic smoking induces increased PRA, which results in increased aldosterone formation, presumably via enhanced generation of angiotensin II. This may partly explain the greater vasoconstrictive reactivity typical of the arteries of chronic smokers.