Forty-eight patients in whom 50 serious anaerobic infections were diagnosed, were treated with metronidazole either orally (33) or rectally (15). Complete recovery occurred in 38 infections, while in 9 there was a satisfactory response, and 3 were considered as treatment failures. Thirteen of the 15 patients treated with suppositories had a complete recovery, while the other patients had a satisfactory response. Serum levels for metronidazole were obtained in 19 patients as well as in 12 volunteers. Similar peak levels, ranging between 10 to 30 mg/l were found after either the oral or rectal route. Sustained high levels were obtained when using the rectal route.