Fluoxetine (FLU) is potent and highly selective serotonin-reuptake inhibitor used in the treatment of major depression. The FLU is apparently the most acute toxic pharmaceutical reported so far for aquatic organisms. Very little is known about possible toxicity of its metabolites. Ingested drugs are excreted to the environment in a biologically active form, either as the parent substance or as an active metabolite. This study was designated to assess the ecotoxicity of FLU and its metabolites. FLU and norfluoxetine (NFLU) were very toxic to applied bioassays with LC50 around 0.5 mg l(-1). The compounds affected only slightly stronger the protozoan Spirostomum ambiguum than the crustacean Thamnocephalus platyurus in the 24 h lethality tests. NFLU was 50% more toxic than FLU in both bioassays. FLU was metabolised in vitro with S9 rat liver fraction. The composition of the reaction mixtures was assessed with HPLC-PAD and MS, and their toxicity was evaluated with the bioassays. The chemical analysis showed besides FLU and NFLU the presence of 4-trifluoromethylphenol, which was much less toxic to the bioassays. Predicted toxicity values were calculated on the base of the FLU and NFLU concentrations in the samples and EC50, and LC50's of the pure compounds. The toxicity of the solutions received during the metabolism of FLU can be predicted based on the concept of concentration addition. The results give the strong indication on the importance of investigation not only parent drugs but also their metabolites.