An inflammatory process was induced by intratendinous injection of bacterial collagenase into the superficial digital flexor tendon (SDFT) of the left thoracic limb of 10 horses. One week later, the tendons in five of the horses (group 1) were treated with glycosaminoglycan polysulphate (GAGPS), and the tendons of the other five (group 2) were treated with saline solution. The horses were euthanased 150 days after the collagenase injections, and samples of the SDFTs were frozen at — 14°C, sectioned at 5 to 7 μm longitudinally and transversely, and stained by the picrosirius red method. Morphometric analysis was used to quantify the organised and disorganised bundles of collagen in the samples from groups 1 and 2. Significantly more organised bundles of collagen were observed in the tendons treated with GAGPS.