It has been observed that the premotion silent period (p. s. p) appears just before a rapid voluntary movement. This phenomenon is believed to be caused by one of the inhibitory discharges from the central nervous system. In the present study the effect of premotion silent period on reactive movement was investigated by comparing the conditions of "p. s. p. present" and "p. s. p. absent." Total of twenty seven healthy males aged 18-23 years were subjected to a series of experiment in which they were asked to exert their muscle strength for two to five seconds at the strength of 15 to 20% of their maximum elbow extension strength. Then they were requested to extend their elbow joint responding to a flashing lamp as quickly as possible. The EMG activities of triceps brachii and biceps brachii were recorded by bipolar surface electrodes. The two conditions of "p. s. p. present" and "p. s. p. absent" were identified from the EMG recordings. The force curve of elbow extension was simultaneously recorded using a resistance strain gauge transducer, and the mechanogram was analyzed on the following items: A) The time from the signal to the onset of force curve (premotor time) B) The time from the onset to the peak of force curve (motor time) C) The time from the signal to the peak of force curve (reaction time) D) The rate of tension rise (peak force strength/motor time, as expressed in kg/sec) The following results were obtained: 1) The mean values of the premotor time were 212 msec in "p. s. p. present" and 195 msec in "p. s. p. absent." (P<0.01) 2) The mean values of the motor time were 89msec in " p. s. p. present " and 93msec in "p. s. p. absent." (P<0.01) 3) The mean values of the reaction time were 302 msec in "p. s. p. present" and 288 msec in "p. s. p. absent." (P<0.01) 4) The mean values of the rate of tension rise were 126kg/sec in "p. s. p. present" and 115 kg/sec in "p. s. p. absent." (P<0.01) It was suggested from these results that the presence of the p. s. p. is related to the shorter motor time, greater rate of tension rise, and prolongated premotor and reaction time, in comparison with the condition in which the p. s. p. is absent.