The effect of spironolactone (300 mg/day) for 28 days onthe systemic blood pressure has been studied in 4 normal and 22 hypertensive subjects in an attempt to distinguish those patients with primary aldosteronism who are suitable for adrenal exploration. No patient had any evidence of primary parenchymal renal disease. Full blood pressure control was achieved in 8 patients. Of these, 2 had renal artery stenosis, 2 microadenomata of the adrenal glands and 4 a solitary adrenal adenoma. Significant reductions in blood pressure were also noted in the remaining 14 patients, but normal values had not been attained in the 28 days of study. Although reductions in blood pressure were usually accompanied by a negative cumulative sodium balance, this was not an invariable feature and there was other convincing evidence to suggest that the hypotensive effect of spironolactone might be mediated through mechanism(s) independent of changes in external sodium balance. The interrelationships between the above observations and the plasma electrolyte, renin and aldosterone concentrations are discussed.