The metabolism of drugs to chemically reactive metabolites may play a pivotal role in the pathogenesis of idiosyncratic drug toxicity. A large number of in vitro studies and a limited number of in vivo studies have demonstrated that many drugs are not toxic per se, but produce toxicity after undergoing enzyme-mediated bioactivation to chemically reactive species. Such reactive species may inflict a toxic insult on the cell either directly or indirectly by acting as a hapten and initiating an immune-mediated reaction. The enzymes responsible for bioactivation have been widely studied, both quantitatively and qualitatively, the most important being the enzymes of the cytochrome P450 (CYP) mixed function oxidase system. CYP enzymes are the most predominant drug metabolising enzymes in the liver and are also present in most other tissues of the body. The diversity of this enzyme system means that a wide range of xenobiotic substrates can be bioactivated by either a single CYP isoform or multiple isoforms of this enzyme superfamily. Other enzymes do, however, play an important role in drug bioactivation. In white blood cells, for example, myeloperoxidase has been shown to bioactivate a wide range of drugs. In other tissues low in CYP activity, prostaglandin H synthase may also be responsible for bioactivation; e.g. in the kidney paracetamol (acetaminophen) toxicity is though to result from activation via this enzyme. The phase II or conjugation enzymes may also be important in the ultimate bioactivation of drug molecules. Whilst activation by these enzymes is, to date, apparently confined to chemicals, most drugs are also substrates for these enzymes and bioactivation by them must remain a possibility.