The purpose of this study was to compare the duration of pulpal anesthesia obtained with 0.5% bupivacaine HCl with 1:200,000 epinephrine to 2% lidocaine HCl with 1:100,000 epinephrine, using the periodontal ligament (PDL) injection. Twenty-four subjects received bilateral PDL injections using mandibular premolar teeth and a double-blind research design. Anesthesia was defined as no response to maximum output of an electric pulp tester. An overall success rate of 35.4% was achieved. Duration of anesthesia obtained with bupivacaine HCl ranged from 4 minutes 59 seconds to 16 minutes 37 seconds with a mean duration of 10 minutes 24 seconds. Excluding one instance in which the duration was three orders of magnitude beyond the norm, duration of anesthesia obtained with lidocaine HCl ranged from 1 minute 43 seconds to 16 minutes 16 seconds with a mean duration of 7 minutes 10 seconds. Postoperatively, all experimental teeth tested vital with no clinically apparent pulpal or periodontal damage. Some postoperative discomfort was experienced at 16.6% of the injection sites.