Injuries to tendons are among the most common injuries in competition horses and represent a significant cause of morbidity and mortality. The present study aimed at an eight -week period ultrasonographic and biomechanical assessments of collagenase induced tendinitis in forelimb superficial digital flexor tendon (SDFT) in donkeys. Four healthy male donkeys were used. 1000 U collagenase type I from Clostridium histolyticum was injected in the center of the SDFT guided by ultrasonography. Ultrasonographical images of the SDFT were recorded prior to injury and 8 weeks after confirmation of tendinitis on a weekly basis. Images were digitized and lesion area to cross-sectional tendon area ratio (LA/TA ratios) was measured. At the end of the study period, the animals underwent tenectomy and tendon samples were evaluated biomechanically. Collagenase induced tendinitis was confirmed ultrasonographically 10 days post-injection. The animals showed swelling and response to palpation, however, no lameness was found in animals within the study period. There were significant differences between the first and the fifth weeks in operated limbs ultrasonographically (p < 0.05). The eight-week period is sufficient to assess the effect of various modalities in tendon healing in the collagenase-induced model in the donkey. The development and expansion of collagenase induced tendinitis until week six after confirmation of tendinitis may disturb findings of the healing effect of various modalities in the tendon, in which tendinitis is still expanding and may mask the healing effect of the modalities used.