To assess the biomechanical effects of intra-tendinous injections of PRGF on the healing Achilles tendon after repair in a sheep model. Thirty sheep were randomly assigned into one of the six groups depending on the type of treatment 10 received (PRGF or placebo) and survival time (2, 4 and 8 weeks). The Achilles tendon injury was repaired by suturing the tendinous edges employing a three-loop pulley pattern. A trans-articular external fixation system was then used for immobilization. The PRGF or placebo was administered on a weekly basis completing a maximum of 3 infiltrations. The force, section and tension values were compared between the operated and healthy Achilles tendons across all groups. The PRGF-treated tendons had higher force at eight weeks compared with the placebo group (p=0.007). Between two and four weeks, a significant increase in force in both the PRGF-treated tendon (p=0.0027) and placebo group (p=0.0095) occurred. No significant differences were found for section ratio between PRGF-treated tendons and the placebo group for any of the time periods evaluated. At 2 weeks PRGF-treated tendons had higher tension ratio compared to placebo group tendons (p=0.0143). Both PRGF and placebo treatments significantly improved the force (p<0.001 and p=0.0095, respectively) and tension (p=0.009 and p=0.0039, respectively) ratios at 8 compared to 2 weeks. The application of PRGF increases Achilles tendon repair strength at 8 weeks compared to the use of placebo. The use of PRGF does not modify section and tension ratios compared to placebo at 8 weeks. The tension ratio progressively increases between two and eight weeks compared with the placebo. PRGF can be used in the clinical setting as a complementary therapy to improve the repair strength of acute Achilles tendon ruptures.