Matthews and Ruthworth described selective gamma efferent fiber paralysis by Procaine applied to the peripheral nerve of cat. In our present study, the posterior tibial nerve was injected with 2% procaine at popliteal fossa in the patients who clinically showed either rigidity or spasticity in the calf muscles or ankle clonus. These clinical signs were decreased or abolished after the injection although voluntary contraction of the muscles and the monosynaptic H-reflex response following electrical shocks to the nerve were preserved. Even at this stage of anesthesia, action potentials of the sciatic nerve set up on stimulating the posterior tibial nerve was detected as well as before the injection through a pair of bipolar needle electrodes inserted in the sciatic nerve at the buttocks. These results indicated that procaine solution of the dosis used in the experiments blocks fibers of small diameter including gamma efferent fibers mainly though it is not effective to the alpha motor and group I afferent fibers, and clinical symptoms of hyperactive stretch reflex i. e. rigidity, spascity and clonus may depend on excessive bias on the muscle spindles in the calf muscles through gamma efferent fibers.