Activity changes were studied on electrodermal activity, heart rate, respiratory rate, and the soleus H-reflex during a mental rehearsal of 500 m speed skate sprint. Subjects were 10 speed skate athletes, ranging in age from 18 to 23 years, who had been fully trained by a image training program which including the same mental rehearsal. They were all highly motivated before each experiment. The mental rehearsal was started by a signal gun recorded by a tape recorder, after a rest period (5 min), relaxation (4 min) and a controlor emotion (2.5 min). Skin resistance of the each subject decreased largely during the mental rehearsal of the skating compared with that during relaxation, and increased gradually after completion of the rehearsal. The relative values of the decrease in all subjects were from 25% to 92% (mean of 65%, p<0.01). Heart rate began to increase prior to the rehearsal of the start, and indicated peak values during the rehearsal. The relative values of the increase were from 14% to 40% (mean of 24%, p<0.05). Respiratory rates also increased obviously during the rehearsal. The relative values of the increase were from 14% to 66% (mean of 43%). The patterns of the change of the soleus H-reflex during the mental rehearsal were classified to three groups as follows; a) a group which did not indicate significant change (4/10), b) a group indicated significant decrease of the H-reflex amplitude (4/10), c) a group indicated significant increase of the H-reflex amplitude (2/10). From the above results, it was suggested that the sympathetic nerve was activated in all subjects, when athletes who were intrested in the experiment and had been fully trained by a image training were sellected for the subjects. On the other hand, the individual variation would be existent among subjects whether the motor command from higher center facilitated or inhibited the motor pool of the spinal cord.