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A number of studies have assessed the physical properties and durability of rotary nickel-titanium files. This study proposes a method that will permit this to be done in a controlled manner. Files of 0.04 taper; 25 mm in length; and in ISO sizes 25, 30, and 35 were each used four times in plastic blocks. The canal in the block had a working length of 17 mm; 30° curvature; 15 mm radius of curvature; size 20 at the apex; and the curve began at 8 mm from the orifice of the canal. The irrigant used was Glyde®. An electric handpiece was configured to rotate at 200 rpm and secured to the testing device, which also held the plastic block. The testing device controlled the depth of penetration at 17 mm; the load was at 8 N and the rate of penetration was at 120 mm/min. Controls were not subjected to use. Torsional moment (N-cm) and angular deflection (degrees) at failure were determined on a Torsiometer/Memocouple. Data (n = 5) were analyzed by analysis of variance. Tukey-Kramer intervals at the 0.05 significance level were determined. Torsional moment significantly increased with an increase in size. Torsional moment also significantly increased for used instruments, compared with controls. Angular deflection had a range of 480 degrees to 810 degrees for the instruments tested.