Aim: This ex vivo study evaluated the accuracy of three electronic apex locators (Root ZX® II, Propex II® and Elements Diagnostic®) in determining working length by comparing their findings with direct measurements. Materials and methods: Forty single-rooted human teeth underwent coronal access and cervical preparation. To determine actual working length, a size 10 K-file was inserted to the point where its tip was visible at the apical foramen under 8× magnification; a rubber stop was placed at the level of the incisal edge, and working length was measured with a millimetre endodontic ruler. The teeth were then mounted on a piece of floral foam soaked in 0.9% sodium chloride, and electronic measurements were made using the foramen identification criterion for apex location of the Root ZX II, Propex II and Elements Diagnostic units. Results: Analysis of variance (ANOVA; P = 0.9987) showed no statistically significant differences between the three electronic apex locators at either tolerance level. Root ZX II had a 93% accuracy rate, Propex II 90% and Elements Diagnostic 91%. No statistically significant differences were found between the locators being tested (P > 0.05), with a safety margin of ± 0.5 mm. Conclusion: The three locators had acceptable accuracy rates for clinical use. Parole chiave: apex locator, endodontics, working length