Aldosterone responses to physiologic doses of known regulatory factors have been studied in 8 patients with treated congestive heart failure under standard conditions of electrolyte balance and controlled body posture. The response of plasma aldosterone, plasma renin activity (PRA), and in some instances angiotension II, to the brief (120 minute) intravenous administration of ACTH (1.25 mug beta1-24 ACTH in 60 minute and 5 mug in 60 minutes), angiotensin II (30 mug) and potassium (30 mEq.), and to 2 hours in the upright posture, was measured. The plasma aldosterone increase was greatest in response to ACTH (mean increment 20.8 ng. per 100 ml.) followed by that with upright posture (mean rise 17.1 ng. per 100 ml.), potassium (mean 6.7 ng. per 100 ml.) and angiotensin II (5.8 ng. per 100 ml.). PRA rose vigorously following 2 hours of upright posture and fell with angiotensin II infusion, but showed no definite change in response to potassium or ACTH. A diurnal pattern of PRA, angiotensin II, and plasma aldosterone on control days was observed in these patients, with higher prenoon levels falling to a nadir at midnight. The electrolyte and hormone response to administered aldosterone (75 mug over 2 hours) was also studied in the same patients. During aldosterone infusions, plasma aldosterone increased (increment range 36 to 110 ng. per 100 ml.), urine sodium excretion decreased, but no significant change in urine potassium excretion or in PRA was observed. It is concluded that small fluctuations in ACTH secretion, as well as change in body posture, produce marked effects on plasma aldosterone in patients with controlled congestive heart failure. Aldosterone responses to ACTH in these patients is similar to that observed in normal subjects.