Conflicting reports of changes in renin activity and angiotensin in congestive heart failure, together with the recent suggestion that the hyperaldosteronism observed is a result of therapy, prompted a reevaluation of the renin-angiotensin system in this condition. Plasma renin activity in seven patients before therapy was in the normal range for upright subjects receiving a liberal sodium Intake. Of six patients for whom renin activity was determined serially during treatment, five showed a marked rise from a pretreatment mean level of 432 ng/100 ml in three hours to 1,402 ng/100 ml in three hours on the second or third day of treatment. This resembles the normal physiological response of the renin-anglotensin system to sodium loss. These findings support the suggestion that in heart failure hyperaldosteronism occurs only after treatment is commenced and indicate a possible therapeutic role for aldosterone antagonists in its management.