Plasma metronidazole was measured following a single intravenous dose of 20 mg/kg (Flagyl 5 mg/ml) in 12 infants less than 1 year of age undergoing abdominal surgery. In all patients sufficient plasma concentrations, well above the MIC values for anaerobic bacteria, were found for at least 16 h. A prolonged half-life was demonstrated in the group less than 8 weeks of age (t1/2/18.4 h). The group over 8 weeks of age demonstrated a t1/2 comparable to that seen in adults (t1/2 7 h).