Using a rigorous extraction procedure and radioimmunoassay we have established that aldosterone 21-sulphate concentrations in adrenal vein specimens are higher than peripheral concentrations. Incubations of homogenates of adrenal glands and adenomata in vitro also produced aldosterone 21-sulphate. Peripheral plasma concentrations in normal and hypertensive subjects are about one-tenth those of aldosterone. Mineralocorticoid activity, as assessed in adrenalectomized rats, is less than 1% of that of aldosterone. [3H]-Aldosterone 21-sulphate administered to normal and hypertensive subjects is excreted in an irregular manner, and in four out of six subjects the major metabolite was an unidentified less polar compound. We have characterized this compound chromatographically and discussed the mechanism which may be involved in its production.