Pharmaceuticals are environmental pollutants that are a major threat to aquatic ecosystems and very little is known about their ecological consequences. In this study growth, survival and behaviour (sociability, activity and boldness) of perch fry (Perca fluviatilis) were examined in order to study the possible effects of exposure to a benzodiazepine anxiolytic drug, Oxazepam, during embryonic development. The study tested following hypotheses: (1) perch growth is affected positively by exposure during embryonic development; (2) early perch survival is affected positively by exposure during embryonic development; and (3) boldness and activity increases while sociability decreases in perch fry exposed during embryonic development. Embryos of naturally spawned perch were exposed to water with two different concentrations of Oxazepam. The embryos were exposed during different parts (24-hour periods) of the embryonic development, because embryos may be more vulnerable at certain times during embryonic development and/or because the exposure at different times can produce different effects. Embryos were also chronically exposed, which is essential for aquatic systems because the influx of pharmaceuticals is more or less continuous. In line with hypothesis 1 treatment with Oxazepam affected growth positively. Similarly, survival increased with Oxazepam exposure as predicted by hypothesis 2. Perch fry exposed to the high concentration and fry exposed late during embryonic development survived better. In addition, as hypothesized in hypothesis 3, perch fry exposed to both concentrations exhibited increased activity and reduced sociability although boldness did not increase. Further studies are required to demonstrate the ecological consequences of Oxazepam in aquatic systems.