Background: Chronic opioids administration could lead to several side effects including morphine tolerance and opioid induced hyperalgesia (OIH). Tolerance and hyperalgesia to opiates reduce their effectiveness in the treatment of severe pain. Although the mechanisms are unclear. Recently, we have shown that repeated morphine treatments induced increases in spinal PKCᵧ and GAT-1 expression. In this study we investigate the BDNF and KCC2 expression through ultra-low dose of naloxone coadministration of morphine.Results: In morphine group, rats received 10 mg i.p. morphine and in treatment group 15ng ultra-low dose of naloxone with morphine for consecutive 8 days. Behavioral tests were performed on day 1 before and after the morphine injection, day 5, 8 (tolerance test) and 10, 48h after last morphine injection (opioid induced hyperalgesia OIH test). A number of rats were sacrificed on day 8 and others on day 10, then expression of BDNF and KCC2 were analyzed by western blot and immunohistochemistry techniques, respectively. Behavioral tests suggested that following 8 days of chronic morphine injection tolerance developed. OIH was shown 48 hours after the last morphine injection. Expression of BDNF significantly was increased and KCC2 downregulated in rats that developed morphine tolerance and OIH respectively. Ultra-low dose of naloxone by decreasing BDNF and increasing KCC2 was able to suppress development of OIH and alleviated morphine tolerance.Conclusions: Our data suggest that BDNF and KCC2 maybe candidate molecules which are involved in tolerance and OIH. Ultra-low dose of naloxone along morphine might be a valuable therapeutic potential for controlling hypersensitivity following chronic morphine administration.