Aim: The aim of the present study was to evaluate in vitro the accuracy of the Mini Apex Locator (Sybron-Kerr, Romulus, USA), Novapex (Forum Technologies, Rishon Le-zion, Israel) and ProPex II (Dentsply-Maillefer, Ballaigues, Switzerland) electronic apex locators compared with the radiographic method to determine the working length. Methodology: The mesiobuccal root canals of 30 extracted human mandibular first molars were used. The actual length was determined visually under magnification. A glass container was filled with sterile 0.9% NaCl solution and the specimens were fixed to a plastic bar using acrylic resin. The apical constriction of the teeth was determined with three electronic apex locators and radiographically. The same procedures were performed 1 mm short of the apical foramen. Each measurement was repeated three times and the mean value computed. These values were tabulated for comparison with the measurements obtained by the electronic and radiographic methods with the actual length. Data were analysed statistically by McNemar test at the 5% significance level. Results: The highest coincidence percentage was found with the ProPex II device (83.0%), followed by the radiographic method (46.7%), Novapex (30%), and Mini Apex Locator (13.3%) in relation to the visual determination of the apical constriction. At 1 mm short of the apical foramen, the highest coincidence percentage occurred with the ProPex II device (80.0%), radiographic method (56.7%), Novapex (26.7%), and Mini Apex Locator (6.7%). There were statistically significant differences between the methods used. Conclusions: ProPex II showed the highest accuracy in determining the working length, followed by the radiographic method, Novapex and Mini Apex Locator. Keywords: apical constriction, electronic apex locator, ProPex II, radiography, working length determination