The effect of exercise on the urinary excretion of albumin, beta 2-microglobulin, renal plasma flow (RPF) and glomerular filtration rate (GFR) was studied in two groups of young patients with mild essential hypertension, one comprising nine untreated patients (1) and one comprising eight patients treated with propranolol (2), and in a group comprising ten normotensive healthy control subjects (3). The urinary excretion of albumin and beta 2-microglobulin, RPF and GFR were measured during four consecutive periods: a control period of 40 min; two exercise periods of 20 min each, with a load of 75 W and 100 W, respectively; and a control period. During exercise group 1 showed an increase in albumin excretion and a decrease in beta 2-microglobulin excretion, while group 2 showed an increase in albumin excretion during heavy exercise only and unchanged beta 2-microglobulin excretion, and group 3 showed unchanged albumin and beta 2-microglobulin excretion. RPF and GFR were reduced during exercise in all groups, most markedly in the hypertensive groups. During both exercise loads albumin excretion was higher and RPF was lower in group 1 than in 3. In group 2 albumin excretion was lower than in 1 during the light exercise load, whereas RPF and GFR were lower during both exercise loads. It is concluded that young patients with mild essential hypertension have both an abnormally high albumin excretion and an abnormally low RPF during exercise. During propranolol therapy urinary albumin excretion was partly normal. The changes in albumin excretion suggest glomerular leakage.