The purpose of this study was to assess the mechanical properties--torsional moment, maximum angular deflection, maximum bending moment, and permanent angular deflection--of four brands of nickel-titanium (NiTi) endodontic file, and compare them with a conventional stainless-steel instrument, both in the presence and absence of sodium hypochlorite (NaOCl). NiTi instruments from four manufacturers were randomly selected and subjected to NaOCl treatment for 12 or 48 h, or not at all. The mechanical properties under test were then measured automatically by a digital torque memocouple. Torsional moment and maximum angular deflection indicate the resistance to torsional fracture of an instrument, maximum bending moment the stiffness of the instrument, and permanent angular deflection the strength of the base alloy. All instruments evaluated complied with or exceeded ADA/ANSI Specification No. 28, with the sole exception of the Maillefer ISO size 40 for torsional moment. JS Dental and McSpadden NiTi files were the most resistant to torsional fracture, but all NiTi files were inferior when compared with stainless-steel files from a previous study. However, NiTi files were superior in flexibility, and Maillefer and Brasseler instruments were the best of the instruments tested. NiTi files also had negligible permanent deformation angles. Furthermore, for all properties tested, NaOCl had no statistically significant effect.