In this study the effects of agonist acting drugs (morphine sulphate, fentanyl citrate and meperidine hydrochloride) on nerve conduction were studied in 43 healthy young volunteers divided into four groups randomly. According to analgesic equivalent doses, the first group received 2 mg morphine sulphate, the second group 0.02 mg fentanyl citrate, the third group 20 mg meperidine hydrochloride, and as control the fourth group received 2 ml of saline. The latencies, amplitudes of the responses and nerve conduction velocities were obtained immediately before and every 5 min after injections up to 30 min. No significant change was observed within or among the morphine sulphate, fentanyl citrate and saline groups whereas in the meperidine hydrochloride group the amplitudes diminished significantly and this finding was still apparent at 30 min. Four of the cases displayed complete blocks. Nerve conduction velocity did not change in the other 6 cases. The significant decrement of the amplitude of the compound nerve action potential in the meperidine hydrochloride group is probably due to local anesthetic-like action of this drug. Morphine sulphate, fentanyl citrate or saline did not show this effect.