en Pain and addiction Neurophysiology/Pharmacology Experimental research article Experimental research article Introduction: Nowadays Opioids are gaining acceptance for the management of moderate to severe and chronic pain. In addition, a number of studies have shown that plasma levels of β-endorphin (βEP) in exercise trained are higher than sedentary rats. In our study we assume that exercise training can reduce pain after withdrawal syndrome in morphine-dependent rats. Methods: Male Wistar rats (250±20 g, N=24) were addicted by morphine sulfate 0.4mg/ml (for 21 days) and animals were submitted to swimming training, five days a week for 8 weeks, in which First 60 minutes for 3 weeks and then 90 minutes in two weeks and at the end 120 min for 3weeks. At the end of each stage of exercise protocol naloxan hydrochloride (3mg/kg.ip) was injected. Tail-flick was used to assess the effect of training on nociceptive threshold. Results: Data showed that swimming aerobic exercise significantly increased pain threshold in trained control and addicted rats (p<0.05) while pain responses did not significantly change in sedentary control and addicted rats. Conclusion: Our results indicate that swimming training has analgesic property in morphine withdrawal-induced hyperalgesia in rats. Therefore, it can be used for the treatment and/or management of painful conditions. Addiction, morphine sulfate, naloxan hydrochloride, nociceptive threshold, swimming training 288 294 http://ppj.phypha.ir/browse.php?a_code=A-10-210-2&slc_lang=en&sid=1 Farzana Yadegari 320031947532846009088 320031947532846009088 No Bu Ali Sina university Ali Heidarianpour heidarian317@gmail.com 320031947532846009089 320031947532846009089 Yes Bu Ali Sina university Farzad Nazem 320031947532846009090 320031947532846009090 No Bu Ali Sina university 320031947532846009091 320031947532846009091 No