Human pharmaceuticals, such as nonsteroidal anti-inflammatory drugs (NSAIDs),are an emerging environmental threat to marine organisms. Bioaccumulation inaquatic organisms along with possible ecotoxicological effects may then be ofconcern, as shown by the inclusion of diclofenac (DCF) in the first watch list bythe EU Water Framework Directive. It is possible to hypothesize that thesecompounds will be pharmacologically active in organisms in which the drugtargets are expressed and functional. In humans, NSAIDs act through inhibition ofcyclooxygenase (COX) conversion of arachidonic acid into prostaglandins. PGsare conserved lipid signaling molecules that mediate a wide array of biologicalfunctions [1]. There have been many reports of PG biosynthesis in a broad rangeof invertebrates, including bivalves [2]. Therefore, modulation of PG productionin marine bivalves following exposure to DCF is worth questioning. The objectiveof this study was thus to investigate whether diclofenac (DCF) affects PG levels inmarine mussels Mytilus galloprovincialis. An experiment was carried out wherebymarine mussels were exposed to DCF for 48 h to 1 and 100 µg/L DCF. A specificand sensitive analytical method using liquid chromatography tandem massspectrometry was developed to quantify DCF accumulation in marine musseltissues. The developed method could also clearly identify and quantifyprostaglandins PGE2, PGD2 and PGF2α levels in mussel tissue and be used toassess their modulation following DCF exposure. Measured concentrations ofDCF in water were relatively close to the nominal concentrations for all exposedaquaria. A low bioconcentration was calculated at 26.4 L/kg. The weakbioaccumulation of DCF observed is consistent with its possiblebiotransformation in the organisms, supported by the detection ofhydroxy-diclofenac metabolites in exposed organisms and water. Basal PGE2concentrations ranged from under the limit of detection (LoD) to 202 µg/kg dw.PGD2 was always found below the LoD. A downward trend in the PGE2concentration was observed between non-exposed mussels and those exposed to1 µg/L DCF, whereas this decrease was confirmed and statistically significant forexposure to 100 µg/L. PGF2a globally ranged from 90 to 518 µg/kg dw. Nodifference was observed for PGF2α levels between controls and exposedorganisms.