Objective-To determine the effect of ultrasound therapy on hydroxyproline (HP) content in superficial digital flexor tendon (SDFT) in horses.Design - Experimental in vivo study.Animals - Eight adult castrated horses.Procedure- The mediocaudal surface of right metatarsal bone in was exposed and after complete separation of SDFT, it was splitted (striking 15 times) in full thickness in longitudinal fashion of 10 cm in length in the mid-tendon area using BP blade no 24. These animals were divided into two equal groups of control and experiment. No treatment was given to control one, whereas; treatment group was subjected to therapeutic ultrasound (Sonopulse 434) regimens 10 minutes daily with intensity of 1 Watt/Esq. at 3 MHz frequency over 14 days. The hydroxyproline concentration was measured by modified spectrophotometery. The collected data was analyzed using student t test at p <0.05 significant level.Results- HP content of severed tendons in both groups was significantly less than normal tendons (P<0.01). Also, the HP content of treatment group was significantly more than the control group.Conclusion and Clinical Relevance- The results of this study indicated that direct application of ultrasound therapy on severely injured or extensive lesion of superficial digital flexor tendon in horses will enhance HP content in the treated tendon.