The aim of this study was to assess the histopathological alterations in the superficial digital flexor tendon (SDFT) of horses with experimentally-induced tendinitis and treated with polysulphated glycosaminoglycan (PSGAG). Tendinitis of the SDFT was induced in two groups (I and II) of five Arabian horses each (both sexes, 2-6 years old) by the intratendineous injection of 1 ml of collagenase (2.5 mg/ml). Seven days after injury, the horses in group I received five intralesional injections of 1 ml of PSGAG (125 mg/ml), every four days, whereas the horses in group II received saline injections of the same volume and frequency. The macroscopic changes observed 150 days after injury included thickening of the paratenon, an increase in vascularity and adhesions to the palmar surface of the SDFT. Histopathological examination revealed an extensive area of fibroplasia and neovascularization, with poorly organized collagen fibers and hypercellular thickening of the endotenon. There were no significant differences in the regeneration of the tendon between collagenase-treated and control groups, thus indicating that the intralesional administration of PSGAG had no beneficial effect in the treatment of experimentally-induced tendinitis. of PSGAG had no beneficial effect in the treatment of experimentally-induced tendinitis.