Purpose: To evaluate effects of melatonin on sodium selenite-induced cataract formation. Methods: Twenty-three Sprague-Dawley rat pups were randomized into three groups. Group 1( n = 9), injected with selenite (s.c.) on postpartum day 10; group 2 ( n = 7), injected with selenite (s.c.) on day 10 plus melatonin (i.p.) on days 8 - 15; group 3 ( n = 7), saline-injected controls. Development of cataract was assessed weekly under a dissection microscope. Rat lenses and serums were analyzed for antioxidant enzymes superoxide dismutase ( SOD) and catalase ( CAT); oxidative stress indicators xanthine oxidase (XO) and malondialdehyde (MDA), a marker of lipid peroxidation; and protein carbonyl ( PC), a marker of protein oxidation. Results: Significant differences ( p < 0.05) were seen in cataract development by the three groups. All rats developed dense nuclear cataract in group 1. Dense nuclear cataract was not observed in group 2: five of seven rats developed minor cataracts, while the other two had clear lenses. In control rats ( group 3), all lenses remained clear. In selenite group ( group 1), lens and serum levels of MDA, PC, and XO were significantly higher and levels of SOD and CAT were significantly lower than those in control group ( p < 0.001). In selenite + melatonin group ( group 2), lens and serum levels of MDA, PC, and XO significantly decreased and levels of SOD and CAT significantly increased when compared with selenite group. Conclusions: Studies with the rat selenite cataract model strongly support the activity of melatonin as an endogenous antioxidant and anticataract agent.