Purpose: The aim was to evaluate the effects of the ultrastructural healing process in tenotomized tendons treated with plasma rich in growth factors (PRGF) in sheep at 2, 4 and 8 weeks. Methods: Forty-two adult sheep were anaesthetized and the Achilles tendon was experimentally tenotomized and repaired. Animals were randomly assigned into six groups. Three groups received an infiltration of PRGF on the repaired area just after repairing the tenotomy and, one of them recived one more injection after one week, and two of them recived the infiltration every week during the following 3 weeks. The other three groups received a placebo injection with saline. At 2 weeks, one PRGF group (PRGF2) and one saline group (S2) were euthanized, and the other four groups were euthanized at 4 weeks (PRGF4; S4) and 8 weeks (PRGF8; S8). Samples of 2 mm in size were taken in a longitudinal direction of the tendon axis from the incision sites which exhibited active regeneration. Tendon samples were fixed with glutaraldehyde and examined with transmission electron microscopy. Collagen fibril diameter and priodicity were measured. The Mann-Whitney U test was perform to analyze the significance between groups. Results: At 2sd week, S2 group showed abundant vascularity, edema and inflammatory cells: B lymphocytes, plasma cells, eosinophils and macrophages. Collagen fibrils were thin, although there were some areas with hypertrophic fibrils. In PRFG2 group, low vascularization and inflammatory cells were seen but mature collagen fiber formation. At 4th week, S4 group still exhibited large areas with abundant vascular components and edema. Highlighting the presence of macrophages and fibroblasts. Collagen fibrils were scattered and anarchic surround the fibroblasts, they were thin, with a few unclear cross-striation masked by the edema. Collagen fibers thicken at specific points atypically, but keeping the striations, forming zones of accumulation of thickened fibers that give rise to microkeloid areas. PRGF4 group showed little sign of inflammation with low number of macrophages and vessels. Tenocytes become tenocyteblasts, considered highly active on the basis of their well-developed Golgi complex and rough endoplasmic reticulum. Tenocyteblasts synthesized tropocollagen with their normal striations and arranged longitudinally to the axis of the tendon. At 8th week, S8 group exhibited fewer vessels and macrophages than S4. It can be highlighted that microkeloids areas significantly increased. PRGF8 group showed a similar state to PRGF4, although collagen fibers were more packed, with complete fiber structure and better distributed. Tenocyteblasts gradually losed their synthetic activity, transform back to tenocytes again perfectly organized and orderly with the collagen fibers. Collagen fibril diameter and priodicity showed significant differences between S and PRGF groups at any time. Conclusions: Electron microscopy of samples obtained from PRGF and S groups revealed obvious differences in the ultrastructural appearance of the tendons. S2 and S4 showed more signs of inflammation than PRGF2 and PRGF4 respectively. Microkeloid formations were observed in S groups. Fibroblast exhibited greater cellular activity and collagen production and collagen fibers were better organized and aligned in PRGF groups showing a more advanced stage of healing.