Background: Diazinon is one of widely used organophosphrous pesticides, can affect both animals and man even after a single exposure. It has a dual toxicity due to acetylcholinestrase inhibition and formation of free oxygen radicals .So, the current work aimed to evaluate the effects of diazinon on the mice testes and the possible protective effect of melatonin. Material and Methods: Male  CD-1 adult mice were divided into 6 groups, (1) control group,(2) melatonin group 10mg/kg,(3) diazinon group (30mg/kg), (4) diazinon group (60mg/kg),(5) diazinon 30mg + melatonin and (6) diazinon 60mg/kg + melatonin. Diazinon was orally administrated 1 and 28 days of treatment, whereas, melatonin was administrated intraperitoneally at a single dose. Testicular damage was examined by using hematoxyline and eosin staining. Results: Diazinon treated groups diminished the plasma acetylcholinestrase activity on day 1 of treatment. Morphometrical analysis showed a decrease in seminiferous thickness (day 1 and 28), with increased testicular superoxide dismutase (SOD) activity (day28). Melatonin          pre-treatment prevented alterations induced by diazinon, except diminution of               acetylcholinestrase  activity. Conclusion: These results suggest that testicular damage observed post-treatment  might be due to elevated concentration of free oxygen radicals (ROS) with diazinon while, pretreatment with  a single dose of melatonin is a potentially beneficial agent to reduce testicular damage     in  adult mice probably by decreasing oxidative stress.