The purpose of the study was to define more precisely ceftriaxone kinetic variations in neonates and infants during the first three months of life. Ceftriaxone pharmacokinetics were studied in 14 newborns and infants with gestational age ranging from 31 to 42 weeks and younger than three months of postnatal age. Ceftriaxone was administered as an intravenous bolus injection over 15 min at a dose of 50 mg/Kg every 24 hours, for a period of 7 to 28 days according to the bacterial diseases. 13 patients had normal renal function and one had a chronic renal insufficiency. Plasma and urine concentrations were measured by a specific HPLC assay. The mean plasma concentration was 180.7 +/- 19.9 ug/ml (mean +/- SD) 30 min after the beginning of the infusion. After 24 h the plasma value was 29.9 +/- 10.0 ug/ml. The mean elimination half-life (t1/2) was 19.9 h, the total clearance (CL) of the drug was 0.38 ml/min/Kg and the volume of distribution (Vd) was 0.32 l/Kg. About 52% of the administered dose was excreted unchanged in urine. In the patient with renal insufficiency we observed t1/2 = 38.9 h and CL = 0.10 ml/min/Kg. Only a slight accumulation of the drug (from 180.7 +/- 19.9 ug/ml to 223 +/- 15.5 ug/ml) was observed during multiple dosing. The volume of distribution and the plasma half-life were significantly correlated (negative correlation) to the postnatal age. There was no correlation between clearance and postnatal age. No side effects were observed in newborns and infants after administration of ceftriaxone.