INTRODUCTION: : In the neonatal period, many factors may interfere with aldosterone production but data currently available are few and controversial. In this study we aim to assess the early urinary aldosterone excretion (UAE) in very low birth weight (VLBW) infants and to identify some possible clinical and biochemical variables that may influence this excretion. MATERIALS AND METHODS: : We conducted a prospective study in 30 VLBW infants. Aldosterone was measured in the first 72 hours of life in the urine and plasma and urinary electrolytes were determined. Demographic and clinical data were also recorded. RESULTS: : The exploratory analysis showed that a significant positive correlation exists between UAE and gestational age and birth weight and that infants with respiratory distress syndrome have higher urinary aldosterone levels than infants without respiratory distress, but only plasma sodium resulted a significant independent factor that negatively influenced UAE at linear regression analysis (coefficient -0.02, 95% confidence interval [-0.03; -0.004]; adjusted R (2) 0.33). CONCLUSIONS: : The renin-angiotensin-aldosterone system of VLBW infants seems to be able, even immediately after birth, to respond to variations of plasma sodium concentrations; measurement of UAE constitutes an interesting method to determine aldosterone production, avoiding blood sampling in neonates so small and frail as VLBW infants.