Fractional sodium excretion (FENa) was measured in 25 neonates during and after treatment with tobramycin. Mean birthweight of infants was 1.83 kg and mean gestational age was 34.3 weeks. Tobramycin therapy was initiated within 48 hours of birth and levels were maintained within the therapeutic range. Mean FENa values were persistently elevated during treatment and decreased to the normal range within 2 days of stopping antibiotics. Blood urea and creatinine concentrations were normal throughout the period of study. Exposure to the aminoglycoside appears to delay the previously described postnatal decline in FENa, most likely reflecting subtle renal tubular dysfunction.