The synthesis and purification of [13C2]p-hydroxyphenyllactic acid from [13C2]p-hydroxyphenylpyruvic acid, the characterization of tert.-butyldimethylsilyl-derivatized tyrosine, p-hydroxyphenylpyruvic acid and p-hydroxyphenyllactic acid, and an isotope-dilution assay for these substances in normal human plasma using gas chromatography-mass spectrometry (GC-MS) are described. Using this method plasma p-hydroxyphenylpyruvate, p-hydroxyphenyllactate and tyrosine levels of 68 +/- 42 ng/ml, 118 +/- 45 ng/ml and 16.6 +/- 6.3 micrograms/ml, respectively, were found in 9 normal adults. Isotope-dilution assays are sensitive enough to determine tyrosine, p-hydroxyphenylpyruvate and p-hydroxyphenyllactate content in normal subjects, and may be useful for studying disorders of tyrosine metabolism, including inborn errors of metabolism, liver disease and ascorbic acid deficiencies.