The increased abundance of cyanobacteria (blue green algae) in many aquatic ecosytems can be correlated to high nutrient loads. Cyanobacteria are known to produce a wide range of toxic secundary metabolites which have been increasingly recognized as animal and human health hazard. In order to withstand chemical stressors, many organisms developed several detoxication systems. This detoxication metabolism consists of three phases: phase I activation (cytochrome P-450); phase II conjugation (glutathione S-transferase, glucuronosyltransferases, glucosyltransferases) and phase III excretion or deposition. In this study the cyanobacterial toxin microcystin-LR, which is the most common toxin of blue-green algae was investigated. We studied the uptake of microcystin-LR, the effects on detoxication systems and the first step of the detoxication metabolism in several aquatic organisms from various trophic levels, like aquatic plants, invertebrates, and fish. The heptapeptide microcystin-LR was absorbed by all investigated organisms. This uptake led to a dose-dependend elevation of the microsomal and soluble glutathione S-transferases. The in vitro formation of a glutathion-toxin conjugate could be shown by HPLC- and MALDI-TOF analysis resulting in a glutathione conjugate with m/z 1302.79. The detoxication of microcystin-LR was comparable in all investigated organisms starting with this conjugation to glutathione catalyzed by the glutathione S-transferases.