The pharmacokinetics of metronidazole was studied in 20 paediatric patients aged 6 weeks and 4 to 14 years, who had trichomonal vaginitis or an anaerobic bacterial infection. The dosage of metronidazole was about 10 or 20 mg/kg b.i.d. orally. The serum concentrations found in children and the corresponding calculated kinetic parameters were similar to those in adults after intake of an equal, weight-related dose. Metronidazole shows rapid diffusion into the saliva with a concentration ratio of about 1.0. This can provide the basis for an efficient non-invasive method of drug monitoring.