A substantial part of the interindividual variability in response to drugs and xenobiotics is related to genetically-determined impairment in drug metabolism. Several drug-metabolising enzymes are polymorphic in humans and often polymorphisms are strongly related to altered drug biodisposition and to the risk of developing adverse effects. Drugs used in general anaesthesia undergo polymorphic metabolism. Among these, halothane is metabolized by cytochrome P450 (CYP) 2E1 and, to a lesser extent, by CYP3A4 and CYP2A6. CYP2E1 also plays a key role in the metabolism of isoflurane, sevoflurane, enflurane and desflurane. CYP2B6, CYP3A4 and CYP2C9 play a relevant role in the metabolism of ketamine. The enzymes involved in the metabolism of thiopental and etomidate remains to be elucidated. Propofol is metabolized mainly by glucuronidation by uridine diphosphate- glucuronosyltransferases (UGTs) and by hydroxylation by CYP2B6 and CYP2C enzymes. The enzymes SULT1A1 and NQO1 participate in later steps in propofol metabolism. All the above-mentioned anaesthetic-metabolising enzymes are polymorphic in man. The present review analyzes the importance of enzymes in the metabolism of anaesthetics and common polymorphisms related to the biotransformation of general anaesthetics and it raises hypotheses on genetic and non-genetics factors related to altered response to anaesthetics that require further investigation. Based on functional relevance and allele frequencies, we identify the most promising targets for the clinical use of pharmacogenomic techniques in anaesthesia to prevent altered pharmacokinetics or adverse drug effects.