The effect of Vitamin B_<12> and aldosterone on nerve conduction was examined as observing action potentials of dorsal and ventral roots isolated from frogs and rabbits. Both B_<12> and aldosterone selectively blocked nerve conduction of sensory nerve. If large doses of those compounds were applied, nerve conduction of motor nerve was also blocked. Therefore those compounds has local anesthetic action. However their blocking action was different from that of procaine in following several features a) latent period existed before initiating blocking action b) progress of blocking action was slower than that of procaine c) lower concentration of those compounds potentiate action potential d) temperature of medium influenced on producing blocking action, decreasing temperature increased threshold, therefore higher concentration of those compounds which block nerve conduction potentiate nerve action potential in lower temperature.