The H-refrex was studied on 22 cases of the normal subject. 1. According to the grade of the voluntary contraction of the gastrocnemius muscle, the silent period of the H-reflex was decreased, i.e. the recovery curve of the H-reflex was shifted to the left, and supanormality was appeard. 2. The shortning of the silent period of H-reflex was also clearly observed on the single motoneuron by voluntary contraction. 3. The increasing of H-reflex amplitude by the surface electrodes was consideced to be caused by the increasing of excitability of the cells, and the increasing of number of excitable cells in the anterior horn of the spinal cord.