A simple clinical assay is described for urinary aldosterone which requires only a limited amount of time on 3 successive days interspersed by 2 overnight incubations. Three ml of a 24-hr urine collection is pre-extracted with methylene chloride to remove free corticoids, hydrolyzed overnight and extracted into methylene chloride. The extract is washed successively with dilute solutions of sodium hydroxide and acetic acid, then with water. Aliquots are incubated with anti-aldosterone anti-serum and :3H-aldosterone overnight, then the bound and unbound aldosterone are separated by dextran-coated charcoal. Healthy volunteers on normal diets ranged from 3.2 to 12.0 μg and after 3 days of salt restriction from 17.4 to 44.2 μg per 24 hr.