Melatonin (N-acetyl-5-methoxytryptamine) is synthesized by the pineal gland during the dark phase of the photoperiod and it is rapidly released to the blood stream where the highest levels reach nanomolar concentrations. Melatonin is a signal that conveys photoperiodic information to synchronize cell physiology with the dark–light cycle. In addition, it is a lipophilic agent that crosses lipid bilayers, and acts as a free radical scavenger, neutralizing hydroxyl and peroxyl radicals among others, preventing lipid membrane peroxidation and apoptosis, and protecting the DNA from the damage induced by free radicals. Moreover, melatonin stimulates gene expression of antioxidative enzymes including superoxide dismutase, glutathione peroxidase, catalase and glutathione reductase. Studies dealing with the antioxidative effect of melatonin on spermatozoa have shown that melatonin significantly reduces the rate of lipid peroxidation in sperm, and it can protect sperm mitochondria from the damage induced by reactive oxygen species (ROS) throughout its effective antioxidative potential.