Urinary aldosterone excretion is commonly determined by assay of aldosterone liberated from the acid-labile metabolite, aldosterone-18-glucuronide (Aldo-18-G), which reflects 5-15% of aldosterone secretion. However, since 3alpha, 5beta-tetrahydroaldosterone (TH-Aldo), the major metabolite, reflects 15-40% of aldosterone excretion, determination of its excretion should usually provide a more accurate index of aldosterone secretion. We have validated a RIA for urinary TH-Aldo and compared its excretion in black and white normal subjects and patients with essential hypertension and primary aldosteronism during consumption of low, normal, and high sodium diets. Urinary TH-Aldo excretion averaged 4.5 +/- 2.0 (mean +/- SD) times that of Aldo-18-G. The ratio of excretion of the two remained relatively constant during low, normal, and high sodium diets. There was no difference in the excretion of TH-Aldo or Aldo-18-G in black vs. white normal subjects or hypertensive patients and no age-related changes in the excretion of either metabolite from 20-60 yr of age. Two of nine patients with primary aldosteronism had normal Also-18-G excretion but elevated TH-Aldo excretion. We conclude that determination of both metabolites increases the diagnostic accuracy for primary aldosteronism.