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 alpha-keto-gamma-methiolbutyrate were studied in normal healthy subjects following oral or intravenous administration of L- or D-isomer of methionine. Dimethyl sulfide concentration in the expired alveolar gas (mean +/- SD) before methionine loading was 2.1 +/- 1.4 ng/dl (N = 23). Maximum concentrations of dimethyl sulfide in oral methionine loading tests were: 56.0 +/- 24.9 ng/dl (D-2g) (n = 4), 66.0 +/- 42.1 (D-1g) (N = 6) and 4.6 +/- 2.6 (L-2g) (N = 4); and in intravenous loading tests: 60.0 +/- 19.9 (D-Ig) (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 alpha-keto-gamma-methiolbutyrate in urine was 0.15 +/- 0.10 microgram/mg Creatinine (mean +/- SD) (N = 5). Postloading values during the initial two hours were 578 and 156 micrograms/mg Creatinine following 3g of D- and 0.20 and 29.7 micrograms/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.