The thesis addresses the problem of chemical mixtures and seeks to contextualise the work in real EU context. This thesis contributes with the new scientific information on tissue-specific, time-, concentration-dependent, multiple stressors influenced geno- and cytotoxicity responses and fluctuations during organism’s recovery. The effects of metal (Zn, Cu, Ni, Cr, Pb, Cd) mixtures and CdSe/ZnS-COOH quantum dots (QDs) as multicomponent chemical stressors and oomycetes Saprolegnia parasitica as a biological stressor on the induction of cytogenetic lesions have been disclosed under controlled laboratory conditions. The measurement of cytogenetic effects in aquatic organisms in situ provided early warning signals on adverse effects of multiple stressors. Exposure to metal mixture at MPC causes time-dependent and tissue-specific responses of geno- and cytotoxicity in S. salar. Recovery of O. mykiss from cytogenetic damage is time-, tissue- and concentration-dependent; fluctuations of cytogenetic lesions is inherent to depuration period. Reduction of MPC of a certain metal in complex mixture markedly increases the levels of geno- and cytotoxicity in erythrocytes of S. salar and R. rutilus. DNA strand breaks in O. mykiss embryos and larvae following exposure to CdSe/ZnS-COOH QDs was indicated; genotoxic potential of S. parasitica in O. mykiss larvae was detected