recent years there has been increasing concern regarding the influx of pharmaceuticals into aquatic environments. One group of pharmaceuticals that has been identified as being of particular concern are the non-steroidal anti-inflammatory drugs (NSAIDs). This study examined the effects of one of the most commonly reported NSAID contaminants, diclofenac, on the physiology of Carcinus maenas. Two environmentally realistic concentrations were examined in this study (10 ng/l and 100 ng/l) at two salinities (35 and 17.5 psu). Female and male crabs were exposed to these two concentrations for 7 days. Following exposure, haemolymph samples were taken from the animals. Measurements were taken for oxyhaemocyanin and lactate concentrations. The effects of this compound on osmoregulatory capacity (OC) in C. maenas were also measured. Exposure to diclofenac had no significant effect on haemolymph lactate and oxyhaemocyanin concentrations suggesting that exposure to this compound does not elicit a stress response in this species. However, exposure to this pharmaceutical did have significant effects on both the OC and the haemolymph osmolality in C. maenas. At both salinities the data suggests that the osmoregulatory ability of C. maenas is impaired when exposed to diclofenac.