To assess the relative importance of the renin-antiotensin system and potassium in the acute regulation of aldosterone secretion in the salt depleted state, normal subjects were infused with an asotonic saline solution containing potassium chloride (KCl) and the responses compared to the infusions of isotonic saline (500 ml/h) and KCl alone (20 mEq K+/h). Subjects were studied recumbent in balance on a 10 mEq sodium/100 mEq potassium diet. The rates of infusing saline and potassium were selected to produce comparable but opposite effects on the levels of plasma aldosterone. As plasma renin activity (PRA) declined and plama potassium increased during the combined saline-KCl infusions, plasma aldosterone levels appeared to be the arithmetic sum of the changes in these parameters. In fact, the calculated changes in aldosterone levels per unit change in PRA or potassium from each infusion alone, correlated well with the observed levels, further supporting an additive relationship. These data support the concept that in the salt depleted state the renin-angiotensin system and potassium are equally important in the acute regulation of plasma levels of aldosterone.