We have investigated the effects of volatile anaesthetics on electron transport chain activity in the mammalian heart. Halothane, isoflurane and sevoflurane reversibly increased NADH fluorescence (autofluorescence) in intact ventricular myocytes of guinea-pig, suggesting that NADH oxidation was impaired. Using pig heart submitochondrial particles we found that the anaesthetics dose-dependently inhibited NADH oxidation in the order: halothane > isoflurane = sevoflurane. Succinate oxidation was unaffected by either isoflurane or sevoflurane, indicating that these agents selectively inhibit complex I (NADH:ubiquinone oxidoreductase). In addition to inhibiting NADH oxidation, halothane also inhibited succinate oxidation (and succinate dehydrogenase), albeit to a lesser extent. To test the hypothesis that complex I is a target of volatile anaesthetics, we examined the effects of these agents on NADH:ubiquinone oxidoreductase (EC 22.214.171.124) activity using the ubiquinone analogue DBQ (decylubiquinone) as substrate. Halothane, isoflurane and sevoflurane dose-dependently inhibited NADH:DBQ oxidoreductase activity. Unlike the classical inhibitor rotenone, none of the anaesthetics completely inhibited enzyme activity at high concentration, suggesting that these agents bind weakly to the ‘hydrophobic inhibitory site’ of complex I. In conclusion, halothane, isoflurane and sevoflurane inhibit complex I (NADH:ubiquinone oxidoreductase) of the electron transport chain. At concentrations of ≈2 MAC (minimal alveolar concentration), the activity of NADH:ubiquinone oxidoreductase was reduced by about 20 % in the presence of halothane or isoflurane, and by about 10 % in the presence of sevoflurane. These inhibitory effects are unlikely to compromise cardiac performance at usual clinical concentrations, but may contribute to the mechanism by which volatile anaesthetics induce pharmacological preconditioning.