Changes in two important mechanical properties, tensile strength and knot strength, and tissue reactions are investigated for four different nonabsorbable sutures, silk, polyester, polyamide 6, and polypropylene, in vivo. Three different sizes (0, 2/0, 3/0) are also used for each suture type. In the experimental in vivo conditions, rats (Rattus norvegicus) obtained from University of Uludag Medical Faculty Experimental Animals Breeding and Research Center are used. The experiments are carried out on the Instron 4301 tensile tester before and after implantation. In the tests, maximum tenacity values are obtained. After all suture materials are broken, a scanning electron microscope and optical light microscope are used to take their pictures. For braided sutures, the encapsulation rate around the suture plays a very important role in the strength and breaking mechanism and axial splits, but controls these parameters for polypropylene sutures. Encapsulation increases the strength and has a greater effect on smaller sizes. A granulomatous inflammation is observed around all sutures in varying degrees of severity.