A simple enzymatic end point method for the determination of L-phenylalanine and 3-phenylpyruvate has been developed. The assay is based on the spectrophotometry determination of NADH formed or degraded in the reaction catalyzed with Thermoactinomyces intermedins phenylalanine dehydrogenase. This method enables the simple and selective determination of L-phenylalanine and 3-phenylpyruvate based on the high substrate specificity of the enzyme. The assay is sensitive in the range of 0.015−0.15 umol of L-phenylalanine and 3-phenylpyruvate.