OBJECTIVE To determine the differences in the onset time and duration of motor block produced by lidocaine 1% and lidocaine 2% via a quantitative and objective method, the measurement of compound muscle action potentials (CMAPs). METHODS Prospective study. METHODS Main operating rooms of a university hospital. METHODS 20 consecutive patients undergoing surgery not requiring intraoperative muscle relaxation. METHODS General anesthesia with unilateral ulnar nerve block was administered. In patients' nondominant (experimental) arms, an insulated block needle was placed adjacent to the ulnar nerve at the wrist while continuous nerve stimulation was delivered to ensure its proper placement. Through this needle, lidocaine 1% or lidocaine 2% was injected. The dominant (control) arm received no injection. RESULTS Monitoring of ulnar nerve-evoked CMAPs was performed simultaneously on both arms. Ulnar nerve function was assessed at baseline and then at 10-second intervals by automatically measuring the amplitude of the evoked CMAPs on a two-channel electromyogram. The mean (+/- SEM) baseline CMAP amplitude prior to injection of lidocaine 1% was 3.10 +/- 0.87 mV and 3.06 +/- 0.89 mV for the experimental and control ulnar nerves, respectively (p = NS); for lidocaine 2%, baseline CMAP amplitude was 3.58 +/- 1.39 mV and 3.70 +/- 1.46 mV, respectively (p = NS). Over the course of the study, the control CMAP amplitude varied by < 12%. At the experimental ulnar nerve, 90% CMAP decrease after injection of lidocaine 1% and lidocaine 2% occurred 7.5 +/- 2 minutes and 5 +/- 1.5 minutes, respectively (p = NS), whereas maximal blockade was achieved after 15 +/- 3 minutes and 11 +/- 5 minutes, respectively (p = NS). Recovery of CMAP to 90% of baseline occurred 184 +/- 31 minutes after injection of lidocaine 1% and 248 +/- 30 minutes following lidocaine 2% (p = NS). CONCLUSIONS The present study describes a technique that can be used in vivo to objectively measure the speed of onset and duration of local anesthetic-induced motor blockade. Although statistically not different, lidocaine 2% demonstrated a faster onset and longer duration of ulnar nerve motor block than lidocaine 1%.