The accumulation of CO2 in the atmosphere and resulting ocean acidification represent a threat to marine ecosystems. While acid-base regulatory capacity is well developed in marine fish, allowing compensation of extra-cellular pH during short-term hypercapnia, the possible energetic costs of such regulation during long-term exposure remain to be established. In this study, juvenile European sea bass (Dicentrarchus labrax) were exposed from 2 days post-hatching to three different ocean acidification scenarios: control (present condition, PCO2-69pt 520 mu atm, pH 7.9), moderate acidification (PCO2\document\ treatments did not affect fish standard metabolic rate (SMR). However, the most severe acidification condition was associated with a significantly elevated maximum metabolic rate (MMR).This was supported by heavier gill system and higher blood haemoglobin concentration. A reduction of maximum cardiac frequency (f(Hmax)) during incremental warming of anaesthetized fish was also observed in both acidification scenarios. On the other hand, the critical oxygen level (O-2crit), the minimum oxygen level required to sustain SMR, did not differ among groups. The increased MMR, associated with maintained SMR, suggests that acid-base compensatory processes, although not increasing maintenance costs, may affect components of bass homeostasis, resulting in new internal physico-chemical conditions. The possibility that these alterations influence metabolic pathways and physiological functions involved in fish aptitude to maximally transport oxygen is discussed.