Human and veterinary pharmaceuticals have been shown to occur in considerably high amounts in sewage treatment plant (STP) effluents and surface waters. The non-steroidal inflammatory drug diclofenac represents one of the most commonly detected compounds. Information concerning possible ecotoxicological risks of the substance are rather scarce. So far there are no data available on its possible effects in fish after prolonged exposure. In order to evaluate sublethal toxic effects of diclofenac in fish, rainbow trout (Oncorhynchus mykiss) exposed to diclofenac concentrations ranging from 1 microg/L to 500 microg/L over a 28 day period were investigated by histopathological methods. In addition, diclofenac residues in various organs were analyzed by means of gas chromatography/mass spectrometry (GC/MS). The histopathological examinations of diclofenac-exposed fish revealed alterations of the kidney such as an hyaline droplet degeneration of the tubular epithelial cells and the occurrence of an interstitial nephritis. In the gills, the predominant finding consisted in a necrosis of pillar cells leading to damage of the capillary wall within the secondary lamellae. The lowest observed effect concentration (LOEC) at which both renal lesions and alterations of the gills occurred was 5 microg/L. In contrast, the light microscopical examination of the liver, the gastro-intestinal tract, and the spleen did not reveal any histopathological alterations neither in diclofenac-exposed fish nor in solvent controls or control individuals. Chemical analysis showed a concentration-related accumulation of diclofenac in all organs examined. The highest amounts could be detected in the liver, followed by the kidney, the gills and the muscle tissue. Dependent on the diclofenac concentration used, the bioconcentration factors (BCF) were 12-2732 in the liver, 5-971 in the kidney, 3-763 in the gills, and 0.3-69 in the muscle respectively. From the present findings it can be assumed, that prolonged exposure in environmentally relevant concentrations of diclofenac leads to an impairment of the general health condition of fish.