The aim of this randomized clinical trial was to compare the anesthetic efficacy of 2% lidocaine with 1:200,000 epinephrine and 3% prilocaine with 0,03 i.u./ml felypressin used for the inferior alveolar nerve block (IANB) in endodontic treatment of adult patients with symptomatic pulpitis in posterior mandibular teeth. 5 patients visiting the emergency clinic of the Faculty of Odontology, Malmö University participated in the study. They randomly received, in a double-blind manner, up to 3,6 ml of either 2% lidocaine (n=4) or 3% prilocaine (n=1) in a conventional IANB. Treatment (pulpotomy) was only initiated after profound lip numbness was obtained. Pre- and perioperative pain, intake of analgesics and anxiety was recorded. Anesthetic success was defined as no or only mild pain (a rating of <54mm on a 170mm Visual Analogue Scale) during treatment. The success rate for IANB using lidocaine was 50% and for prilocaine 100%. The material size was insufficient to enable statistical analysis. The few patients selected for this pilot study were insufficient to assess any difference in anesthetic efficacy between lidocaine and prilocaine for IANB in endodontic treatment of adult patients with symptomatic pulpitis in a posterior mandibular tooth. Further data collection is needed to make conclusions about possible superiority of any of the tested substances. A sample size of 57 test subjects in each group would give a power of 80% to detect a difference of 25% in success rate. The protocol appears to be suitable for use in a larger-scale study.