Intravenous metronidazole was administered, either by continuous or intermittent infusion, to 20 patients with infections involving anaerobic bacteria; 14 of the 20 patients were changed to oral administration of metronidazole for completion of therapy. Six of eight patients with infections derived from oropharyngeal bacterial flora were cured; the addition of ampicillin was required in one patient, however, because of an incomplete response to metronidazole. Eight of eleven evaluable patients with infections derived from bowel flora were also cured by metronidazole or metronidazole plus an aminoglycoside. Of 93 anaerobic bacteria isolated before therapy, 89 were susceptible to 16 micrograms or less of metronidazole per ml. Mean plasma levels of metronidazole were 27.6 +/- 11.4 micrograms/ml in patients receiving continuous infusions of drug and 19.9 +/- 10.7 micrograms/ml (trough) in patients receiving intermittent infusions. Two patients developed peripheral neuropathy during therapy. Metronidazole is an effective agent for the treatment of anaerobic infections. Because metronidazole is not active against facultative and aerobic bacteria, the addition of a second antimicrobial agent may be required for the treatment of mixed anaerobic-aerobic infections.