The excretion and metabolism of radiolabelled S-methyl-L-cysteine (150 mg orally) was studied in three male volunteers. The major route of excretion was the urine with 55.9% of the administered radioactivity being voided over three days (33.3% during the first 24 h). Faecal excretion was relatively unimportant (c. 1.4%). The remaining radioactivity was either slowly excreted in the urine over the next 21 d (35S-labelled compound) or exhaled (14C-labelled compounds). Metabolism occurred via the pathways of S-oxidation, N-acetylation and deamination. Extensive degradation of the molecule was observed with the production of large amounts of inorganic sulphate and CO2. The options for the sequence of metabolic degradation of the amino acid derivative are discussed.