Aim: To evaluate the effects of melatonin on antioxidant enzyme levels and histopathologic changes in dizocilpine (MK-801)-induced psychosis model rat testis. Methods: A total of 24 adult male Wistar-Albino rats were divided into three groups with 8 in each. Group Ⅰ was used as control. Rats in Group Ⅱ were injected with MK-801 (0.5 mg/kg body weight i.p. for 5 days). In addition to MK-801, melatonin (50 mg/kg body weight i.p. once a day for 5 days) was injected into the rats in Group Ⅲ. The testes were harvested bilaterally for biochemical and histopathological examinations. Antioxidant enzyme activities, malondialdehyde, protein carbonyl and nitric oxide (NO) levels in tes-ticular tissues were analyzed using spectrophotometric analysis methods. Histopathological examinations of the testes were also performed. Results: MK-801 induced testicular damage, which resulted in significant oxidative stress (OS) by increasing the levels of antioxidant enzymes. The malondialdehyde, protein carbonyl and NO levels were increased in testicular tissues of rats. Treatment with melatonin led to significant decrease in oxidative injury.Administration of melatonin also reduced the detrimental histopathologic effects caused by MK-801. Conclusion:The results of the present study showed that MK-801 cause OS in testicular tissues of rats and treatment with melatonin can reduce the harmful effects of MK-801.