Attaining profound local anesthesia is frequently difficult. Standard block or infiltration injections often are not sufficient; a supplementary injection is often necessary. The purpose of this study was to examine the effectiveness of the periodontal ligament injection in patients who did not have adequate pulpal anesthesia. Information was obtained by questionnaire after 120 periodontal ligament injections. The frequency and rapidity of onset of anesthesia was determined as well as the factors that might affect the technique. The following conclusions were obtained from this study. -Mandibular molars required supplementary anesthesia more frequently than other types of teeth. -Injecting under strong backpressure was important; the greatest frequency of success was attained when injecting under pressure. Injecting without strong pressure on both mesial and distal surfaces resulted in the lowest frequency of anesthesia. -Onset of anesthesia was generally very rapid, usually immediate. -The length and gauge of needle were unimportant in attaining anesthesia. -Rejection was frequently successful if the first periodontal ligament injection failed. -The overall frequency of success in attaining anesthesia with this injection was 92%. This rate included situations in which the injection was administered more than once. -The most critical factor was to inject under strong resistance. This necessitates wedging the finger supported needle into the interproximal space between root surface and bone and applying maximum pressure to the syringe handle.