The composition of the mitochondrial inner membrane and uncoupling protein such as adenine nucleotide translocator (ANT) contents are the main factors involved in the energy wasting proton leak. We have shown that this leak was increased by glucocorticoid treatment under non phosphorylating conditions. The aim of this study was to investigate mechanisms involved in glucocorticoid-induced proton leak and to evaluate the consequences in more physiological conditions (between states 4 and 3). Isolated liver mitochondria, obtained from dexamethasone-treated rats (1.5 mg/kg/day), were studied by polarography, western blotting and high-performance thin-layer chromatography. We confirm that the dexamethasone treatment in rats induces a proton leak in state 4, that is associated with an increased ANT content, without any change in membrane surface or lipid composition. Between states 4 and 3, dexamethasone stimulates ATP synthesis by increasing both the mitochondrial ANT and F1-F0 ATP synthase content. In conclusion, dexamethasone increases mitochondrial capacity to generate ATP by modifying ANT and ATP synthase. The side effect is an increased leak in non phosphorylating conditions.