A flow injection enzymatic method for the spectrophotometric determination of l-phenylalanine has been developed. l-phenylalanine is deaminated in the presence of l-amino acid oxidase and the keto acid formed is made to react with borate to give a coloured enol-borate complex that can be detected at 282 nm. Catalase is added to the catalyzed reaction to prevent the keto acid being destroyed by the hydrogen peroxide generated. Kinetic determinations are performed by measuring the change in absorbance between 2 and 4 min. The proposed procedure, involving both merging-zones and stopped-flow techniques, can be applied to the quantitation of l-phenylalanine between 10 and 260 mg l(-1). Detection limit and R.S.D. are 1.1 mg l(-1) and 3.0%, respectively.