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With the view of evaluating the relative utilization of methionine isomers, the pulmonary exhalation of dimethyl sulfide and methyl mercaptan and the urinary excretion of a-keto-7-methiolbutyrate were studied in normal healthy subjects following oral or intravenous administration of L-or D-isomerof methionine. Dimethyl sulfide concentration in the expired alveolar gas (mean ± SD) before methionine loading was 2.1 ± 1.4 ng/dl (N=23). Maximumconcentrations of dimethyl sulfide in oral methionine loading tests were: 56.0 ± 24.9 ng/dl (D-2g) (n=4), 66.0 ± 42.1 (D-lg) (N=6) and 4.6 ± 2.6 (L-2g) (N=4);and in intravenous loading tests: 60.0 ± 19.9 (D-lg) (N=4) and 4.2 ± 2.1 (L-2g) (N=3), respectively. The changes in methyl mercaptan in the expired alveolar gas were small and were disproportional to the changes in dimethyl sulfide following administration of both isomers. Preloading concentration of a-keto-7-methiolbutyrate in urine was 0.15 ± 0.10 Mg/mg Creatinine (mean ± SD) (N=5). Postloading values during the initial two hours were 578 and 156 Mg/mg Creatinine following 3g of D-and 0.20 and 29.7 Mg/mg Creatinine following 3g of L-methionine ingestion, In view of the results obtained, significant amounts of D-methionine seem to be metabolized through the transaminative pathway of methionine metabolism.