Melatonin is a free-radical scavenger and antioxidant. Roentgen irradiation of testis (animal model formed of albino rats) was associated with destruction and depletion of the germinal epithelial cells. In roentgen-irradiated, melatonin-pretreated animals, these changes were markedly ameliorated. Thus administration of melatonin before irradiation can protect testis from some of the destructive effects of roentgen irradiation.