Plasma renin activity (PRA), plasma renin substrate (PRS), plasma renin concentration (PRC), and plasma aldosterone concentration (PAC) were measured in normal full-term newborn infants and infants up to 22 days after birth (36 in all). As compared to the control, PRA and PAC levels were significantly higher (p less than 0.001) in cord blood, and in 2 to 6 and 12 to 22 day old infants. The high values of PRA in 2 to 6 day old infants were due to the high PRS values. In cord blood and in 12 to 22 day old infants, however, the high levels of PRA reflect elevation of PRC. No correlation was found between PRA, PAC and serum potassium in cord blood and in 2 to 6 day old infants, but there was a significant correlation between the parameters (r = 0.72 and p less than 0.02) in 12 to 22 day old infants, indicating that at this age, as in healthy adults, the renin-angiotensin system and serum potassium seem to be the major stimuli for aldosterone secretion.