Abstract: Objective To investigate the effects of chronic morphine administration on pain behaviors in young rats and to explore the mechanisms involved. Methods Sixteen SD young rats of 3-4 weeks were randomly divided into control and morphine administration groups. Young rats received saline (1 mL/kg) or morphine (10 mg/kg) subcutaneously. Each regimen was given once daily for 14 days. Pain behaviors were examined on day 1, 3, 5, 7, and 14 before the daily drug administration. Western blot was used to examine the expression of glutamate decarboxylase 65 (GAD65) in the spinal cord dorsal horn on day 14 after the last drug administration. Results Following 14 days of morphine administration, mechanical hyperalgesia was induced in young rats. Compared with control group, the mechanical withdrawal threshold of morphine group significantly decreased on days 3, 5,7 and 14. Chronic administration of morphine downregulated the expression of GAD65 in the spinal cord dorsal horn of young rats. Conclusion Chronic morphine administration could induce mechanical hyperalgesia in young rats, and the downregulation of GAD65 in the spinal cord dorsal horn might play a critical role in the molecular mechanisms of morphine-induced hyperalgesia.