The main purpose of this study was to compare the cyclic fatigue resistances of the three different nickel-titanium files in 2.5% sodium hypochlorite solution and distilled water at physiologic intracanal temperature (35 degrees C). Three NiTi files such as One Curve (OC) (Micro-Mega, Besancon, France), OneShape (OS) (Micro-Mega), and Protaper Next (PTN) X2 (Dentsply Sirona, Ballaigues, Switzerland) were tested for cyclic fatigue resistance inside artificial stainless steel canal block with curvature 60 degrees and 5 mm radius. The block was fixed inside a hot water bath of distilled water or 2.5% sodium hypochlorite and the temperature was set at 35 +/- 1 degrees C. The number of cycles to fracture data were recorded and all the fracture surfaces of files were evaluated by using scanning electron microscopy. The number of cyclic to fracture data were analyzed statistically via Kruskal-Wallis and Mann-Whitney U tests. OC showed the highest cyclic fatigue resistance at distilled water while PTN X2 showed the lowest cyclic fatigue resistance at 2.5% sodium hypochlorite. When cycling fatigue test medium were changed from distilled water to sodium hypochlorite solution, the resistance of the files to cyclic fatigue were decreased. SEM analysis confirmed that all scanned samples were fractured due to cyclic fatigue. The OC rotary instruments have more cyclic resistance than OS and PTN X2. The sodium hypochlorite solution reduces the resistance of the rotary instruments. C1 [Ertugrul, Ihsan F.] Pamukkale Univ, Dept Endodont, Fac Dent, Denizli, Turkey.