It was observed in the preliminary study that there was diminution of the H reflex in the calf muscles with hypoactive ankle jerk after gamma fibre blocking with procaine injected in the tibial nerve. Although no remarkable change on motor power of the muscle was seen. The present investigation deals with mechanism of this diminution of the H reflex and with regulation of motor unit discharge during voluntary contraction after gamma fiber blocking. 1. Diminution of the H reflex. Amplitude of H reflex response which had been diminished after gamma fibere blocking was recovered by voluntary contraction of the calf muscles. There was little change observed in the latent period of the H reflex between the experimental states with and without voluntary contraction. Excitability of the motoneurone measured by the H reflex recovery curve with conditioning and test stimulus indicated that there was a marked increase of excitability of the motoneurone during voluntary contraction of the muscles. These results suggest that the diminution of the H reflex after gamma fibre blocking is due to decreased motoneurone excitability which is a result of diminished afferent impulse to the motoneurone from the spindle. On the other hand, augmentation of the H reflex response during voluntary contraction is due to increased motoneurone excitability where descending impulse to the motonerone from upper motor neurone compensate the loss of afferent impulse from the spindle. 2. Effect of gamma blocking upon veluntary contraction. There was never observed the démblée type of discharge at the onset of contraction and could not do a quick contraction of the muscle. There was seen prolonged reaction time on the E. M. G. although no decrease of motor power detected after gamma bloking. This may indicate again that there is decrease of motoneurone excitability after gamma blocking and it takes a little time to recover this decreased excitability by impulse from upper motor neurone at the onset of voluntary contraction.