The histopathologic changes caused by microvascular anastomosis were studied in 178 femoral arteries of rats. While in all vessels the tissue surrounded by the suture loops at the anastomosis showed the strangulating effect of the suture with the process of repair being evident on the endothelial and adventitial surfaces, there were certain differences in the changes between patent and thrombosed vessels in the areas adjacent to the anastomosis. A study of the incidence of various tissue changes showed that endothelial integrity is important for patency of the anastomosis.