Plasma renin activity was determined in 25 healthy, full-term, newborn infants aged 1 day to 9 weeks. High values were found, the mean level at 1-2 days of life (24.8 +/- 8.4 ng/ml/hr, SE) being significantly higher than the mean levels at 7-9 days (5.8 +/- 1.5) and at 4-9 weeks (8.1 +/- 1.3) (P less than 0.05). No correlation was found between plasma renin activity and systolic blood pressure, hematocrit, creatinine clearance, serum sodium, or serum potassium. Plasma renin activity (log values) was inversely correlated with sodium intake (r = -0.58) or with urinary sodium (r = -0.44), and positively with urinary osmolality (r = 0.67). The correlations reached higher coefficients if only infants aged less than or equal to 9 days were considered. In addition, vasopressin was measured by radioimmunoassay in the urine. The daily excretion was lower in newborn infants (9.4 +/- 1.6 ng/m2/day, SE, at 1-2 days of postnatal life) than in healthy children (37.1 +/- 5.6), and was significantly correlated with creatinine clearance (r = 0.69), but not with urinary osmolality.