Nowadays, tramadol hydrochloride is frequently used as a pain reliever, and for the treatment of premature ejaculation. Decreased semen quality was noted in chronic tramadol users. The present study aimed to elucidate the effects of tramadol on the testicular functions of adult male rats. A total of 40 albino adult male rats were divided into control and tramadol groups, with 20 rats for each group. Rats of the tramadol group were subcutaneously injected with 40 mg/kg three times per week for 8 weeks. The control group received normal saline 0.9%. Blood samples from each animal were obtained. Plasma levels of different biochemical substances were determined. Nitric oxide was measured in testicular tissue samples. Those samples together with epididymal tissue samples were processed for histopathological examination. Tramadol significantly reduced plasma levels of luteinizing hormone, follicle‐stimulating hormone, testosterone and total cholesterol, but elevated prolactin and estradiol levels compared with the control group. In addition, tramadol increased the testicular levels of nitric oxide and lipid peroxidation, and decreased the anti‐oxidant enzymes activities significantly compared with the control group. The tramadol group showed decreased sperm count and motility, and numbers of primary spermatocytes, rounded spermatid and Leydig cells. Immunohistochemical examinations showed that tramadol increased the expression of endothelial nitric oxide synthase in testicular tissues. The present study showed that tramadol treatment affects the testicular function of adult male rats, and these effects might be through the overproduction of nitric oxide and oxidative stress induced by this drug.