We investigated the role that mitochondrial proton leak may play in the glucocorticoid-induced hypermetabolic state. Sprague^Dawley rats were injected with dexamethasone over a period of 5 days. Liver mitochondria and gastrocnemius subsarcolemmal and intermyo¢brillar mitochondria were isolated from dexamethasone-treated, pair-fed and control rats. Respiration and membrane potential were measured simultaneously using electrodes sensitive to oxygen and to the potential-dependent probe triphenylmethylphosphonium, respectively. Five days of dexamethasone injection resulted in a marked increase in the basal proton conductance of liver mitochondria, but not in the muscle mitochondrial populations. This e¡ect would have a modest impact on energy expenditure in rats.