Single-dose kinetics of 0.1 mg/kg intravenous diazepam, 10 mg/kg oral antipyrine, and 300 mg oral phenytoin were followed in healthy subjects before and after 400 mg metronidazole twice a day for 5 days. When data before metronidazole dosing were compared with those after metronidazole dosing, there were no changes in total plasma clearance of diazepam (0.53 and 0.65 ml/min/kg), antipyrine (39.0 and 38.0 ml/min/kg), or phenytoin (0.56 and 0.55 ml/min/kg). Plasma t 1/2s and volumes of distribution of the three drugs tested were not affected by metronidazole, but urinary excretion of 4-hydroxyantipyrine decreased after metronidazole dosing. There was no change in the elimination of phenytoin as its hydroxylated metabolite after metronidazole. It is concluded that, at therapeutic concentrations, metronidazole does not significantly inhibit oxidative drug metabolism.