Changes in tensile strength and knot security of commonly used surgical suture materials during the postoperative period were evaluated by implanting sterile suture loops in the subcutaneous tissue of rats and rabbits. Tensile strength testing after varying periods of implantation showed that of the materials studied, only Dacron maintained its initial knot security and tensile strength. Silk and cotton showed moderate loss of strength even in two weeks. Of the absorbable sutures, polyglycolic acid sutures were superior in tensile strength and markedly superior in knot security as compared to catgut. Both materials showed progressive loss of strength reaching minimal values at about three weeks.