The working length is considered the distance between a coronal reference point and an apical level, in order to obtain the instrumentation and obturation limit [1]. This mensuration is one of the main factors in endodontic treatment success [2]. Previous determination of working length is critical to cleaning, shaping and obturation of radicular canals and has to be carefully measured [3]. The objective of this study is to evaluate the accuracy of three apex locators using four irrigant solutions to determine the working length of root canals. Material and methods: thirty unirradicular teeth were used. Teeth were randomly divided into three groups, in order to establish the working length, using different apex locators: Raypex 6, Endo EZE and RootOR. Four different irrigant solutions were used combined with the apex locators: saline solution (0.9% sodium chloride), 2% chlorhexidine digluconate, 18% EDTA and 5.25% NaCLO. Real working length (RWL) was determined using a #15 K-file in each specimen until the end of the file was visualized in stereoscopic microscopy. Scores (%) were attributed to each difference found between the working length gave by the apex locators and the RWL. Influence of Irrigant Solutions and Apex Locators in Working Length DeterminationResults: there was a significant difference between the RWL and that one found with the apex locators Raypex 6 and RootOR; however, Endo EZE did not show statistical differences. The four different irrigant solutions did not show statistically differences between then. Conclusions: the precision to estimate the working length with Raypex 6 and RootOR was affected by the different irrigant solutions in the root canal; Endo EZE was the most accurate apex locator to estimate the RWL.Abstract