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Potassium and angiotensin II are major regulators of aldosterone secretion. To assess which of these stimuli is the more potent, we measured aldosterone, potassium, and angiotensin II responses to the oral converting enzyme inhibitor captopril in five patients with resistant congestive heart failure during digoxin and furosemide maintenance therapy. In spite of a positive cumulative potassium balance and a clear-cut rise in plasma potassium, aldosterone levels in plasma and urine declined in parallel with levels of angiotensin II. When captopril treatment was later withdrawn in three patients, angiotensin II and aldosterone levels increased in parallel, while plasma potassium remained steady. The results show that under these study conditions, angiotensin II is more potent than potassium in regulating aldosterone in patients with heart failure.