The hydroxy metabolite of metronidazole was found to be more active against 21 strains of Gardnerella vaginalis than the parent compound and less affected by culture in carbon dioxide. After 400 mg oral metronidazole (Flagyl) plasma concentrations of the two agents were below the minimum inhibitory concentrations (MICs) for most G vaginalis strains tested. With 2 g metronidazole the plasma concentrations exceeded the MICs of the more sensitive strains. Even with the lower dose of metronidazole clinically useful concentration of metronidazole and its hydroxy metabolite were present in the urine. Urinary excretion of these compounds may contribute to the efficacy of metronidazole in the treatment of vaginitis associated with G vaginalis.