The synthesis of DL-tyrosine containing isotopic nitrogen has been effected is good yields by a four-step process: the Erlenmeyer azlactone synthesis as modified at Herbst and Shemin, in 52--66% yield; breaking the ring by mild hydrolysis with acetone and water, in 71--94% yield; acid hydrolysis to obtain the hydroxyphenylpyruvic acid, in 48--53% yield; and catalytic hydrogenation of the substituted pyruvic acid in the presence of isotopic 73--90% yield;N15-acetyl-DL-tyrosine has been resolved by the use of d-alpha-phenylethylamine, to give yields of 35--60% of the salt of the L-isomer, from which N15-L-tyrosine was isolated is 77% yield. This new method of resolution involves less manipulation, thus is less time-consuming, and gives slightly better yields than the methods reported heretofore;The isotopic tyrosine was oxidized in vitro by guinea pig liver homogenate, and alanine subsequently isolated from the reaction mixture as the azobenzene-p-sulfonate salt;Isotopic analysis of the isolated compound showed almost all of the N 15 from tyrosine to be contained in the alanine derivative;Isotopic tyrosine was fed to guinea pigs on scorbutigenic diets with and without supplements of ascorbic acid and pteroylglutamic acid, and the urine and feces examined for isotope content. These experiments indicated that the vitamins apparently have little effect on the disposal or fate of the amino-nitrogen of tyrosine;A simple but satisfactory method has been described for collecting isotopic nitrogen gas for analysis on the mass spectrometer.