Metronidazole is being extensively used in neonates for the prevention and treatment of anaerobic infections. However, detailed pharmacokinetic studies are not available. Thirty neonates receiving intravenous metronidazole 7.5 mg/kg at eight-hour intervals were studied with serial blood levels. The levels increased up to the third day of treatment and then reached a plateau. Neonates less than seven days of age showed higher levels than older infants. Septicemic neonates, however, showed continually rising levels. The half-life of metronidazole was 22.3 to 22.5 hours, approximately twice that in adults. This study indicates that neonates handle the drug differently than adults and may require it in lesser doses or at greater time intervals.