Plasma aldosterone (PA) and urinary aldosterone (Aldo-U) concentrations were studied in 123 patients with primary (essential) hypertension during basal (1 h supine rest), upright and frusemide (80 mg orally) stimulated conditions, and were related to urinary sodium and potassium excretions, supine and sitting blood pressure (BP) and the relationship to plasma renin activity (PRA). As controls, 120 normotensive subjects, matched for age and sex, were investigated identically during strictly defined out-patient conditions. No differences regarding the different mean PA levels, urinary electrolyte excretion or the urinary sodium: potassium ratio were observed between the hypertensive and the normotensive populations. However, the hypertensive subjects had significantly higher mean Aldo-U excretions than the controls. Correlations between PA and the corresponding PRA were consistently significant in the normotensive control group but weak to non-existent in the hypertensive subjects. No relationships at all could be found between the different PRA and Aldo-U values in the hypertensive population but significant correlations were noted in the control group. These findings point to a disturbed function of the renin-angiotensin-aldosterone (RAA) system even in primary hypertension.