One of the most important factors contributing to poor quality semen and thus reproductive functions of male animals has been reported to be oxidative stress. This condition develops due to some or all of these factors: a) increased production of oxygen derived oxidants commonly known as ROS; b) reduced antioxidant defense because of the reduced synthesis of antioxidant molecules or c) a decreased intake of natural antioxidants such as vitamin E, A, C, carotenoids and selenium. Mammalian spermatozoa membranes are rich in polyunsaturated fatty acids (PUFA) and are sensitive to oxygen-induced damage mediated by lipid peroxidation. In order to maintain vital reproductive ability, a protective mechanism against oxidative stress has great importance. To protect spermatozoa from oxidative stress, both spermatozoa and seminal plasma contain antioxidants superoxide dismutase, glutathione peroxidases, catalase, paraoxonase 1, glutathione, α-tocopherol, ascorbic acid, glutathione, thoredoxine and albumin. Seminal plasma has a higher concentration of antioxidants than any other biological fluid, including blood serum, which compensates for the low antioxidative capacity of spermatozoa themselves. The main source of ROS in immature forms of spermatozoa is considered to be proximal or distal protoplasmic droplet, located below the head or on the tail in some of spermatozoa. Such forms of immature sperms are not enduring and are infertile. In bovine and ram semen, ROS are generated primarily by dead spermatozoa. Contaminating leukocytes in the ejaculate are an important source of ROS in human and stallion semen.