Neomycin, an antibiotic which is primarily active against the aerobic gut flora and hence reduces the endogenous production of ammonia, is a well-recognised form of treatment for acute or acute on chronic hepatic encephalopathy. This study suggests that metronidazole may be a useful alternative or even adjunctive treatment for such patients. Theoretical and practical justifications for the use of this drug are presented. The results of a week's prescription of each drug have been assessed by changes in clinical and biochemical criteria, including electroencephalograms and arterial ammonia sample. In the treatment of a series of 11 mildly or moderately, and seven severely affected, patients with histologically confirmed cirrhosis, metronidazole is shown to be as effective as neomycin.