The purpose of this study was to compare the accuracy and efficiency of two electronic apex locators to those of a conventional technique of determining working length with radiogjraphs. Data were collected from 53 patients with a total of 96 canals treated by dental students. No significant difference was noted in the accuracy of the two apex locators. The readings from these units exactly agreed with the working lengths established by the students and faculty in 34% of the canals and were within 1.0 mm of the length in 80 to 86% of the canals. The data seem to indicate that apex locators have the potential for reducing the number of radiographs required if used before the working length radiographs are taken. Working lengths should be verified radiographically, because electronic apx locators were not accurate in every case.