A direct structural identification, and quantitative assessment below the 50 nmol/ml level, of the full pattern of renally excreted metabolites is made possible by C-13 NMR measurements of untreated urine samples when stable isotope-labelled (C-13) drug analogues are administered to humans. The full potential of the new ex vivo NMR approach is exemplified by a study, for a group of volunteers, of S-carboxymethyl-L-cysteine metabolism. The metabolic sulphoxidation pathway fo S-caroxymethyl-L-cysteine in man, accepted so far, needs to be profoundly revised on the basis of the C-13 NMR results.