Plasma renin activity (PRA) and plasma aldosterone concentration (PA) were measured in 20 healthy infants 2-12 months of age, after taking a commercial modified cow's milk formula with 8.2 mmol/l sodium for at least five days, and after at least five days on a home-made, undiluted acidified cow's milk formula with 12.6 mmol/l sodium. The mean sodium intakes were 1.20 and 1.87 mmol/kg of body weight per day respectively. The arithmetic means for the corresponding PRA were 6.16 and 1.08 ng/ml . h (difference not significant) and the corresponding arithmetic means for PA were 603 and 355 pg/ml, the difference being highly significant (P less than 0.01). It is concluded that one has to consider the sodium intake in the milk formula when studying plasma aldosterone in infancy.