Reactive oxygen species (ROS) play in the male reproductive system important physiological functions in cell signaling, spermatogenesis and sperm maturation in epididymis. The influence of various factors, e.g. environmental, could be the reason for oxidative stress. This can lead to the exposure of cells to the toxic effects of many oxidants such as O•⁻₂, O₃, H₂O₂, •OH. The source of ROS may be spermatozoa, which also, due to the composition of the cytoplasmic membrane lipids and great amount of mitochondria are particularly susceptible to oxidative damage. Oxidative stress can damage their membranes, DNA, inhibit sperm motility and reduce their fertilization ability. This dual effect of ROS confirms the unique role of antioxidant enzymes (such as SOD, CAT and GPX) and non-enzymatic (e.g. GSH, vitamins A, E, C, transferrin) responsible for maintaining adequate levels of ROS in the male reproductive system.