Pharmaceuticals represent a diverse collection of thousands of bioactive chemicals used in human and veterinary medicine. The increased consumption, together with the recent development of more sensitive analytical techniques, has identified these as emerging contaminants in the aquatic environment. According to many investigations pharmaceuticals do not cause acute toxic effects in organisms when released in the environment. However, many independent studies agree that chronic exposure and more specific endpoints should be used in risk assessment of these compounds. We thus investigated the effects of exposure to environmentally relevant concentrations of the antiepileptic drug carbamazepine (CBZ) on Mediterranean mussels (Mytilus galloprovincialis) by considering the existing knowledge about the therapeutic and side effects of this drug on humans. To do so we analysed: (a) six consolidated biomarkers related primarily to oxidative stress; (b) cAMP levels and protein kinase A (PKA) activities; (c) mRNA expression of MXR-related genes. MXR proteins are involved both in the cAMP pathway and in the protective response of organisms towards xenobiotics. Mussels exposed to 0.1 or 10microg CBZ per liter water for 7 days showed a 60% and 80% reduction in haemocyte lysosome membrane stability, respectively. Moreover, increased neutral lipid and lipofuscin accumulation in the digestive gland, and lipid peroxidation in gills and mantle/gonads were observed. Also glutathione S-tranferase and catalase activities were increased in digestive gland and mantle, while no increase in primary DNA damage was observed. In agreement with the mode of action of CBZ in humans, exposure resulted in a significant reduction in cAMP levels and PKA activities in digestive gland, gills and mantle/gonads of mussels, and lowered the mRNA expression of genes encoding three different MXR-related transporters in the same tissues. Our data indicate that CBZ, at concentrations found in the environment, affects the Mediterranean mussel by acting on specific biochemical pathways that are evolutionarily conserved.