The influence of several aldosterone antagonists on sodium, potassium, and chloride balance was investigated in a subject with an isolated deficiency of aldosterone, but with otherwise intact adrenal cortical function. Spironolactone, progesterone, and 17α-hydroxyprogesterone have previously been reported to induced a natriuresis in normal subjects and in patients with adrenal cortical insufficiency only when concurrently treated with desoxycorticosterone or aldosterone. These agents were demonstrated to produce sodium loss in the subject studied even in the absence of a potent mineralocorticoid. Although qualitatively similar, less pronounced sodium diuresis occurred in the present study. It is suggested that the observed effects of the administered steroids reflect an antagonism of the renal tubular effects of the glucogenic steroids, which in the subject with aldosterone deficiency appear to serve a significant, but not totally compensatory, mineralocorticoid function. It is concluded, therefore, that the action of these antimineralocorticoid agents is not confined to an antagonism of the more potent salt-retaining steroids, but under certain circumstances may exert their renal tubular effects in the presence of other steroids possessing a relatively minor mineralocorticoid activity.