Since the efficacy of oseltamivir carboxylate (OC) as the active metabolite of Tamiflu has been demonstrated against influenza viruses and even against the avian influenza virus (H5N1), millions of Tamiflu treatment courses are stockpiled worldwide. This was done not at least to follow the recommendations of the World Health Organization (WHO) to cope with a viral influenza pandemic. Concentrations up to 26-32 microg l(-1) OC in river catchment areas in the United States and in the United Kingdom had been predicted recently for a pandemic case, assuming an apparent persistence of the Tamiflu metabolite. Such concentrations may involve the risk of generation of antiviral resistance. But there is still a lack of data concerning the stability of OC in a surface water environment. Under this aspect these predictions have to be validated with concrete facts about the environmental fate of OC. In this article we summarized the results of three different daylight exposure experiments with OC in different waters under sterile and non-sterile conditions simulating shallow water processes at the latitude of approximately 52 degrees N. Using a river water solution containing 50 microg l(-1) OC under non-sterile conditions a half-life time of 17.8 days was observed. Direct photolysis plays no or only a negligible role for the decomposition of OC. Degradation of OC seems to occur as a combination of microbial metabolism and indirect photolysis.