Free flap transfer has become a common technique in plastic surgery for closure of extensive defects but is technically demanding. EPC can promote neovascularization and are therefore a possible therapeutic target to minimize complications. These cells have shown their proangiogenic potential in various studies in different animal models and settings. While some induce angiogenesis via paracrine effects, others are directly incorporated into newly formed vessels. The following study explored the hypothesis, that EPC increase the neovascularization in microvascular transferred free flaps and improve their wound healing. The rat was used as an animal model to transplant the free flaps using a previously described operation technique (162). The technique had to be modified to achieve sufficient flap survival rates. A preliminary study investigated the effect of different times of sample taking on vascular architecture and histological parameters. As most changes occurred on the seventh post-operative day, this became the time interval for the main study. Here, the animals were given EPC s.c. or intraarterially and the control group received none. Samples were taken for histological and SEM examination of the vasculature. Measurement of wound tensile strain could not be performed as the wounds proved to fragile. Histologically there was a significant increase in vessel numbers of 39 % (EPC intraarterially) and 45 % (EPC s.c.) in the treatment groups and no difference was observed regarding the application method. No significant differences were observed when measuring the width of the cutaneous layers and the proliferation rate. The SEM analysis showed no significant differences in the vascular architecture (vessel diameter, vessel density, interbranching distance, branching angle), which might be detectable at earlier stages. Likewise, there was no significant difference in the appearance of pillars and sprouts. This is possibly due to the fact, that SA occurs earlier, and IA is just starting at the seventh day of wound healing. This study could confirm the positive effects of EPC on neovascularization in free flap transfer as shown in previous studies in other applications. Future studies are necessary to determine the mode of the proangiogenic effect of the EPC and their effect on tensile strength of free flap wounds. In veterinary medicine there is need for further research of species-specific EPC and their effects in the respective animals.