The mechanisms of the aldosterone-inhibiting and antihypertensive actions of heparin and certain other sulfated mucopolysaccharides were investigated in 13 hypertensive patients. N-formyl chitosan polysulfuric acid (RO 1-8307) was used rather than heparin because of its low anticoagulant activity. RO 1-8307 lowered aldosterone secretion in one patient with proved and one with probable primary aldosteronism; this indicates that the mucopolysaccharide does not inhibit aldosterone secretion via the renin-angiotensin mechanism. In six hypertensive patients RO 1-8307 caused a fall in the secretion of aldosterone and its probable immediate precursor, 18-hydroxycorticosterone; therefore it does not act at the last step in the synthesis of aldosterone. RO 1-8307 produced a prolonged clinical and biochemical remission in the two patients considered to have primary aldosteronism but did not influence blood pressure in five of seven patients in whom excessive aldosterone was probably not a major factor in the hypertension. It is concluded that the antihypertensive action of RO 1-8307, and perhaps of heparin as well, is largely due to suppression of aldosterone secretion.