Plasma renin activity (PRA) and aldosterone concentration (PA) were measured in 20 healthy infants from 1 to 12 months of age during normal (3.03 mmol/kg/d) and high (5.20 mmol/kg/d) potassium intake. PRA did not change between regimes. PA rose significantly from 618 pg/ml during normal to 1136 pg/ml during high potassium intake. It is concluded that the potassium intake should be taken into consideration when studying plasma aldosterone in infancy.