Summary MATERIALS AND METHODSPlasma 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 f 13) (P < 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 = -OM), and positively with urinary osmolality (r = 0.67). The correlations reached higher coefFicients if only infants aged 5 9 days were considered.In addition, vasopressin was measured by radioimmunoassay in the urine. The daily excretion was lower in newborn infants (9.4 f 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. SpeculationThe postnatal changes of plasma renin activity were significantly correlated only with parameters of sodium balance and with urinary osmolality, so that one can speculate that early after birth, the renin-angiotensin system plays a preponderant role not only in sodium homeostasis, but also in water balance.Plasma renin activity (PRA) (8,16,23,27) and angiotensin I1 concentration (7) are relatively high in human newborn infants. PRA decreases progressively over weeks and months (8,16,22); a similar trend is followed by aldosterone secretion rates (4), plasma aldosterone (4), and exchangeable sodium (19).The cause of increased activity of the renin-angiotensin system during the neonatal period is not clear. The low sodium intake, the low blood pressure, the hypovolemia, and the increased sympathetic activity ,have been invoked as possible factors. On the other 'hand, no correlation was found between PRA and plasma aldosterone. either in cord blood (10, 26) or in newborn infants . . , aged 2-3 days (26), suggesting that angiotensin I1 is not a predominant regulatory factor controlling aldosterone secretion at this age.In an effort to gain insight into the interplay between renin, sodium, and renal function during the neonatal period, we have studied the relationship between PRA, sodium intake and output, urine osmolality, and glomerular filtration rate (GFR) in hea6hy, full-term newborns of different ~ostnatal aees. In addition. vaso-" pressin was determined in the urine, because limited data are available concerning this hormone in human newborns. 7