1. This article surveys the role of stable isotope labelling and mass spectrometric detection in the recognition and characterization of drug metabolites.2. Applications of the so-called isotope cluster technique, useful for both recognition of drug-related compounds and elucidation of their structures, are presented.3. The use of deuterium-labelled g.l.c./mass spectrometric derivatization reagents to facilitate the determination of drug metabolite structures via‘peak shifts’is illustrated.