Using a one-compartment model, the pharmacokinetic disposition of furosemide was studied in eight premature and term neonates with fluid overload. Following a single intravenous injection of furosemide (1 to 1.5 mg/kg), multiple blood samples were obtained from a heelstick or an arterial catheter and analyzed for furosemide by gas liquid chromatography. The mean (+/- SE) apparent volume of distribution was 829.2 +/- 118.9 ml/kg; t1/2 was 7.7 +/- 1.0 hour; elimination rate constant was 0.102 +/- 0.013/hour and plasma clearance was 81.61 +/- 14.98 ml/kg/hour. Compared to the disposition of furosemide in normal adults. AVd is almost fourfold greater in the neonate with an eightfold prolongation in plasma t1/2, an eightfold decrease in Ke1, and a twofold decrease in plasma clearance. Neither gestational and postnatal age nor birth weight correlated with the pharmacokinetic variables. No significant change in reserve bilirubin-binding capacity after an intravenous dose of furosemide was noted. Slow elimination of furosemide may partly explain the prolonged diuretic and saluretic effect of furosemide in the neonate.