An accurate and relatively simple radioimmunoassay for the determination of aldosterone concentration in peripheral plasma has been developed, 0.5-2.0 ml plasma with added [1,2-3H]aldosterone is extracted with dichloromethane. Purification of the extract is achieved by thin later chromatography in the system benzene-acetone 1:1. Recovery of [1,2-3H] aldosterone is 58 +/- 6 (SD)%. Bound and free fractions are separated by dextran-coated charcoal. The intra-assay reproducibility is 8.8% and the inter-assay reproducibility varies from 11.4-16.1%. The sensitivity of the assay for a 5 ml plasma sample can be put at 0.2 ng/100 ml. Normal values determined in 52 healthy children of different age groups are presented. Furthermore the aldosterone stimulating effect of low sodium diet (17 children), severe and prolonged vomiting (19 children) and synthetic ACTH (10 children) has been studied by our modified method.