Objective- The aim of this study was to assess the clinical and ultrasonographic alterations in the superficial digital flexor tendon (SDFT) of horses with collagenase-induced tendinitis. Design- Experimental study. Animals- Five clinically normal adult horse. Procedures- Two thousand units of collagenase were injected in the center of the SDFT of forelimbs of each horse under ultrasound guidance. Clinical examinations were performed to evaluate heat, response to palpation, presence of swelling, and lameness grade. Ultrasound images of the SDFT were recorded prior to injury and at 1, 3, 5, 7, and 14 days after injection. Images were digitized, and percentage of lesion, the echogenicity score (ES), and the fiber alignment score of the fibers (FS) at the maximum injury zone (MIZ) were measured. The sum of the core lesion from 8–20 cm distal to the accessory carpal bone was also measured. Results- Collagenase resulted in anechoic core lesion involving 20%-35% of the tendon cross section at MIZ. There were not any significant difference in the size, ES, and FS between all the days except between the first day and the other days. There were no significant differences between observations made of each structure in the left and right forelimbs of the same horse. Conclusions and Clinical Relevance- Collagenase-induced injury is ultrasonographically similar to naturally occurring injury and causes the same tendinitis in both left and right forelimbs and also in different horses. Tendon lesions using collagenase injection represent an experimental model for tendon injury in research trials.