A validated gas chromatographic-mass spectrometric method for quantitation of phenylalanine and tyrosine in serum is described. Quantitation of phenylalanine and tyrosine with a non-labelled non-endogenous internal standard, l-2-chlorophenylalanine, compared favourably with isotope dilution mass spectrometric quantitation. The 95% reference ranges for phenylalanine, tyrosine and the phenylalanine-tyrosine molar ratio in neonate cord blood serum were determined by isotope dilution mass spectrometry and were found to be 77.1-144.7, 33.3-109.3 μmol/l and 1.1-3.0, respectively.