Shaliutina-Kolešová A., Rui N., Ashtiani S., Rodina M., Cosson J., Linhart O. (2018): Oxidative stress and antioxidant enzyme defence system in seminal plasma of common carp (Cyprinus carpio) and rainbow trout (Oncorhynchus mykiss) during spawning season. Czech J. Anim. Sci., 63, 78-84.Assessment of seminal oxidative stress indicators is emerging as a valuable prognostic tool in assisted reproductive technology. We investigated levels of oxidative stress caused by reactive oxygen species (ROS) and the antioxidant enzyme defence system comprising superoxide dismutase (SOD), glutathione reductase (GR), and glutathione peroxidase (GPx) in seminal plasma (SP) of common carp Cyprinus carpio and rainbow trout Oncorhynchus mykiss over the course of the spawning season. Oxidation was determined in lipids and proteins by assessing thiobarbituric acid reactive species (TBARS) and the 2,4-dinitrophenylhydrazine carbonyl groups, respectively. Levels of SOD were assessed by the autoxidation of pyrogallol. Determination of GR and GPx was based on the rate of NADPH oxidation. We observed clear alterations in lipids and proteins over the course of the spawning season in both species. The highest levels of TBARS and CP were recorded late in the season. SOD was not significantly altered in either species. The activity of GR was higher in carp SP late in spawning compared to other times (P < 0.05), while in rainbow trout, GR significantly increased (24.13 ± 2.8 mU/mg protein) in mid-season. A significantly lower GPx activity (9.18 ± 1.32 mU/mg protein) was found in rainbow trout SP early in the spawning season, but no significant differences in GPx were observed over the course of the season in carp. These results provide further understanding of the role of fish SP antioxidants and present new data on the oxidant and antioxidant balance in SP during the spawning season that may be of value in the development of methods for artificial reproduction of teleost species.