Newborns undergo an isotonic contraction of their extracellular fluid (ECF) volume in the first days of life. This is reported to be secondary to a sodium diuresis proportional to the serum ANP level. To determine if the postnatal urine sodium/potassium ratio can be used as a marker in VLBW infants for plasma ANP levels, eleven babies without sepsis or history of asphyxia were studied. Their mean ± SD BW and GA was 1284 ± 145 g and 29.8± 1.1 wks, respectively. Serum and spot urine, sodium, potassium and creatinine and serum ANP levels were determined on day 4,7,10,14 and 19 of life. Sodium and potassium values were determined via routine potentiometric methodology. Creatinine was determined enzymatically with Kodak Ektachem slides. ANP was determined via RIA. Daily weight as well as total volume and sodium intake were recorded. The mean ± SD ANP levels fell from 47.58± 31.1 pg/ml on day 4 to 37.7 ± 36.7 pg/ml on day 10 with little further change through day 19. The mean ± SD fractional sodium excretion fell from 3.5 ± 2.3% on day 4 to 0.74 ± 0.6% on day 10 and remained < 1.0% thereafter. The mean ± SD urine sodium/potassium ratio fell from 5.49 ± 2.56 on day 4 to 1.25 ± 1.11 on day 10 with little further change through day 19. Serum ANP levels significantly(p<.02) correlated with fractional sodium excretion. Further, the ANP values were significantly (<.01) correlated with the log of urine sodium/potassium ratio but not with total volume or sodium intake/day. We conclude that serum ANP concentrations correlate directly in VLBW infants with renal sodium regulation. Similarly, the log urine sodium/potassium ratio can be used as a predictor of serum ANP levels.