Urinary albumin and beta 2-microglobulin excretion rates were measured by radioimmunological methods in 60 children and adolescents at rest and during physical exercise. The geometric mean of the albumin excretion rate was 4.4 (microgram/min)/m2 at rest and rose to 7.9 (micrograms/min)/m2 during exercise (P less than 0.001), while the geometric mean of beta 2-microglobulin was 31.9 (ng/min)/m2 at rest and 26.2 (ng/min)/m2 during exercise. These results indicate that exercise-induced proteinuria is of a glomerular leaking type. The albumin excretion rate was not dependent on the age or sex of the subjects. The exercise-induced albuminuria correlated weakly but significantly with the maximal blood pressure (r = 0.27; P less than 0.05) and with the physical fitness of the subjects (r = 0.28; P less than 0.05).