1. Plasma aldosterone concentration and urine aldosterone excretion were studied before and during sustained diuresis in six patients with gross congestive heart failure, under conditions of fixed sodium and potassium intake and strict control of body posture. Simultaneous measurements of plasma renin activity, plasma corticotrophin and electrolytes were made to assess the relative importance of these factors in the regulation of aldosterone secretion before and during treatment of congestive heart failure. 2. During the pre-treatment phase aldosterone levels were normal or raised, but with acute diuresis fell to unmeasurable levels in most cases. This depression in aldosterone tended to coincide with peak natriuresis. Later in the diuretic phase aldosterone values increased often to very high levels as dry body weight was attained. 3. With few exceptions plasma renin activity fluctuations paralleled those of plasma aldosterone, whereas corticotrophin levels remained largely within normal limits and plasma electrolytes did not change appreciably. 4. The results suggest that the renin—angiotensin system is the important regulator of aldosterone secretion before and during diuretic treatment in patients with gross congestive heart failure.