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Background. The Achilles tendon is the most injured tendon in humans. There is a close relationship between changes in the structural composition of the extracellular matrix and tendon tissue related to age and menopause because estrogen deficiency can lead to decreased collagen synthesis and a consequent decrease in the strength and elasticity of tendons, indicating a greater propensity for injuries and ruptures. However, there is still a lack of scientific evidence on the relationship between decreased estrogen levels during menopause and the involvement of this structure. This study sought to evaluate the effect of melatonin treatment on the Achilles tendon of ovariectomized Wistar rats and investigate the influence of the treatment on tissue structure, extracellular matrix, and biomechanics. Materials and methods. Light microscopy and polarized light microscopy were used to observe the tendon's morphology using HE and toluidine blue dyes. Tissue biomechanics aspects were tested to assess possible functional changes in organ. Results. Significant changes arose from the ovariectomy processes and melatonin treatment, including histological changes, GAG profile alterations, and biomechanical properties. Conclusions. Melatonin reversed some effects of ovariectomy, but also led to changes in the tendons. Further studies are needed to better understand the relationship between these factors and to identify dose-dependent effects as well as their relationship with the pathophysiology of tendinopathies. [ABSTRACT FROM AUTHOR]