OBJECTIVE The aim of this study was to formulate a regression equation for calculating working lengths in primary molars using EAL (Root ZX and Ipex) for teeth with and without resorption and to evaluate, in vivo, the accuracy of the electronic apex locators in determining working lengths in primary teeth with and without resorption in comparison with Ingles radiographic method. STUDY DESIGN The study was a cross-over study. Fifty children undergoing pulpectomy in primary mandibular molars (total 150 root canals) with and without resorption were evaluated. The actual length of each root was measured with Ingles radiographic method followed by the use of two generations of apex locators. The distances were measured with radiographs, apex locators and working lengths tabulated. Frequency distribution of working length from radiographic apex and their percentages were calculated. Pearson's correlation coefficient was used to assess the closeness of the measurements to radiographic apex with or without resorption followed by formulating regression equations. RESULTS It was observed that 131 (87.33 %) of the 150 readings with Root ZX, and 122 (81.33 %) of the 150 readings with IPEX were within 0.0-1.0 mm of the radiographic readings. Root ZX gave 96.77 % (30) and IPEX gave 90.32 % (28) of 31 acceptable readings (within 0.0-1.0 mm of the radiographic apex) in resorption cases. Measurements with apex locator and radiographic methods were highly correlated. CONCLUSION Apex locators are reliable when compared with Ingles radiographic method and can enhance the safety of treatment in specific situations such as presence of root resorption. In case of non-availability of apex locators, the equations given in our study can be of use.