Plasma atrial natriuretic peptide (ANP) and aldosterone concentrations, and plasma renin activity (PRA) were measured by radioimmunoassay concurrently in 20 healthy full term infants, in cord blood, at 24 hours after birth and on the 4th day of life. ANP and aldosterone increased significantly at 24 hours and was persistently elevated on the 4th day of life. PRA remained unchanged during the first four days of life. In cord blood, ANP concentrations were correlated with aldosterone concentrations (r = 0.49, p less than 0.05) and hematocrit (r = 0.58, p less than 0.02). At the 24 hours of life, plasma ANP concentrations were correlated with weight loss observed on the 4th day of life (r = -0.70, p less than 0.005), while the percentage changes in plasma aldosterone concentrations were correlated with percentage changes in systolic blood pressure (BP) (r = 0.49, p less than 0.05). These findings suggest that during the early newborn period ANP and aldosterone act as an integrated system which has a role in regulation blood pressure and intravascular volume homeostasis.