A number of athletes abuse anabolic steroids to improve muscle performance. It has been shown that supratherapeutic doses of these substances can effect on opioidergic system. It is well known that aerobic exercise has hypoalgesic effects, which proposed to be related to activation of opioidergic system. In this study, the chronic effects of nandrolone decanoate on nociception, β-endorphin and met-enkephalin levels in exercised rats were evaluated. For our aims, both Sedentary (S) and trained (T) male rats received daily injections of nandrolone (15 mg/kg, 5 times/week) or vehicle. After two weeks, nociceptive threshold was estimated by tail flick test, and Circulating levels of β-endorphin and met-enkephalin were determined using ELISA method. Results showed that two weeks of swimming exercise training, were significantly increased nociceptive threshold and serum opioid peptides levels. Administration of nandrolone decanoate was significantly decreased nociceptive threshold and opioid peptides levels in trained group; despite its decline, the effect of nandrolone decanoate in sedentary group was not statistically significant. In conclusion, these findings revealed that two weeks administration of nandrolone decanoate can reduce the plasma opioid peptides level as well as exercise induced hypoalgesia in trained rats.