This randomized double-blind crossover trial investigated the efficacy and discomfort associated with slow (60 seconds) and rapid (15 seconds) inferior alveolar nerve blocks (IANB) using 2.0 ml of 2% lidocaine with 1:80,000 epinephrine in securing mandibular first molar, premolar and lateral incisor pulp anesthesia in 38 healthy adult volunteers. Episodes of maximal stimulation (80 microA) without sensation on electronic pulp testing were recorded. Injection discomfort was self-recorded by volunteers on 100 mm visual analogue scales. Data were analyzed by McNemar, Friedman, Wilcoxon Signed Ranks, and paired t tests. Slow IANB produced more episodes of no response to maximal pulp stimulation than rapid IANB in molars (220 episodes versus 159, p < 0.001), premolars (253 episodes versus 216, p = 0.003) and lateral incisors (119 episodes versus 99, p = 0.049). Slow IANB was more comfortable than rapid IANB (p = 0.021).