The antipsychotic drug quetiapine (QUT) has been frequently detected in sewage treatment plants. However, information on the fate of QUT in aquatic environments and its behavior during UV treatment is limited. In this study, QUT is shown not to be readily biodegradable in the Closed Bottle Test and the Manometric Respirometry Test according to OECD guidelines. The main biotransformation product (BTP) formed in the tests, a carboxylic acid derivative, was identified by means of high-resolution mass spectrometry. This BTP is presumably a human metabolite and showed higher detection rates than QUT in a river sampling campaign conducted in northern Germany. UV elimination kinetics of QUT at different initial concentrations (226.5, 45.3, 11.3, and 2.3 μmol L(-1)) were faster at lower initial concentrations. All seven phototransformation products (PTPs) could be still identified at initial concentration of 11.3 μmol L(-1). The photolytic mixture generated after 128 min of photolysis of QUT was not better biodegradable than QUT. Initial UV treatment of QUT led to the formation of several additional BTPs. Four of them were identified. The bacterial cytotoxicity and genotoxicity before and after phototransformation of QUT in a modified luminescent bacteria test (LBT) and the umu-test (ISO/FDIS 13829) showed cytotoxic effects in the LBT for QUT. Furthermore, PTPs had similar cytotoxic effects on luminescent bacteria. The umu-test did not reveal any genotoxic activity for QUT or PTPs. In conclusion, the release of QUT into sewage treatment plants and aquatic environments could result in the formation of a main BTP. Additional UV treatment of QUT would lead to the formation of additional BTPs. Moreover, treatment did not result in lower toxicity to tested organisms. In conclusion, UV treatment of QUT should be considered critically as a potential treatment for QUT in aquatic systems.