Urine of four hyperammonemic patients who were treated with large amounts of sodium benzoate were analyzed. Gas chromatographic and gas chromatograph-mass spectrometric analysis showed the presence of large amounts of benzoylalanine in addition to much hippuric acid. Benzoylalanine was identified by comparison with an authentic standard by means of its mass spectrum and the gas chromatographic retention time of its trimethylsilyl derivative. This abnormal metabolite is thought to be derived from the conjugation of benzoyl CoA with alanine. It is suggested that the excretion of benzoylalanine results from a reduction in the liver free glycine level caused by hippuric acid synthesis.