BACKGROUND: A mixture of local anesthetics such as lidocaine and bupivacaine has frequently been used in clinical practice. The rationale behind this is to take advantage of lidocaine's rapid onset and bupivacaine's perpetuation in anesthesia. The purpose of this study was to examine the changes in the onset and recovery of nerve blocking action exerted by the different combinations of these two in the mixture. METHODS: Isolated sciatic nerve preparations obtained from adult male Sprague-Dawley rats were used in this study. Recordings of A-fiber compound action potentials (A-CAPs) were made at the end of the isolated nerve while single pulse stimuli (0.5 msec, supramaximal intensity, 2 Hz) were applied to the opposite end of the nerve. Seven different composition of lidocaine-bupivacaine mixtures were prepared (0 : 6, 1 : 5, 2 : 4, 3 : 3, 4 : 2, 5 : 1, 6 : 0 vol./vol.), where basal concentrations of lidocaine and bupivacaine were 0.2% and 0.05%, respectively. Amplitudes of A-CAPs were measured before, during and after perfusion of mixture solution. The time needed for A-CAPs amplitude to decrease to 10% of the basal value after starting perfusion (onset time) and that needed to reach to 50% of the basal value after ceasing the perfusion (recovery time) were measured. RESULTS: With increasing concentration ratios of lidocaine to bupivacaine in the mixture as mentioned above, the following onset and recovery times were obtained (6.0 +/- 0.3, 5.6 +/- 0.3, 6.0 +/- 0.5, 8.3 +/- 0.5, 7.3 +/- 0.6, 7.8 +/- 0.3, and 10.8 +/- 0.8, minutes; 38 +/- 4, 63 +/- 12, 87 +/- 19, 100 +/- 13, 104 +/- 18, 137 +/- 27, and 157 +/- 18 minutes, respectively). CONCLUSION: Onset times were, in general, exponentially decreased with the increase in the lidocaine concentration. However, recovery times were lineary increased with the increase in the bupivacaine concentration. So, it should be kept in mind that rapid onset can only be obtained with the expense of substantial reduction in the duration of local anesthetic effect of the mixture, and vice versa.