Objective: The present study evaluated the influence of NaOCl and of the cutting action on the fatigue resistance of ProFile rotary NiTi endodontic instruments employed for shaping 10 curved root canals and then tested in fatigue using an artificial canal with radius of 4mm and angle of curvature of 45º. Methods: Sixty new instruments, 20/.06, 30/.04, 25/.04 e 20/.04, were randomly divided into a Control Group, Groups 1 and 2, with 20 samples in each group (five of each size). Mesial roots of 50 mandibular extracted molars, with average radius of 4,1mm and angle of 32,9º were selected and randomly divided between the groups. In Group 1, de-ionezed water was employed as irrigant, while in Group 2 a solution of 5.25% NaOCl was used. Results: It has been observed that the instruments of Group 2 were statistically faster in shaping the root canals than those in Group 1 (Student's t test p< 0,05). The fatigue resistance of the instruments in Groups 1 and 2 was significantly lower than in the Control Group, indicating that the cutting action was the critical factor to reduce the number of cycles to failure (Student's t test and Kruskal-Wallis p< 0,05). This parameter decreased with the increase in instrument size in all groups. There was a tendency of the instruments of Group 2 to show greater fatigue resistance than those of Group 1, although these results were not statistically significant. Conclusions: The use of NaOCl during shaping of curved root canals reduced the time spent in the instrumentation and this can be an important factor to increase the useful life of the instruments.