It has been reported that activation of autonomic effectors during mental simulation of voluntary motor actions(motor imagery:MI)may be explained by two different factors, i.e., functions of preparation or anticipation of actual exercise(motor anticipation)and the central motor programming/planning which acts during actual motor action(motor programming). This study was designed to clarify how these factors participate during MI, utilizing two mental tasks with high mental stress, i.e., MI and mental arithmetic(MA). Several autonomic effectors' responses were compared between MI of a 500 m speed skating sprint and MA. Subjects were eight 18 to 25 year old young male speed skate athletes, all of them could easily and vividly imagine a 500 m speed skating sprint. Duration of the MI ranged from 35 to 38 sec and these were very close to each subject's actual best record(means of absolute differences were less than 0.6 sec, i.e., less than 1.7% relatively). A significant decrease of skin resistance(SR), increases of heart rate(HR)and respiration rate were observed in both MI and MA when compared to each control resting level(excluding one subject for respiration rate during MI). SR decreased during MI(mean and SD of 8 subjects:45.9 ± 17.7%)and MA(39.7 ± 16.8%), with no significant differences between MI and MA(t=1.29, by paired t-test). HR increased significantly above control values in MA(10.3 ± 4.3%)and MI(44.3 ± 18.8%). However, the increase during MA was significantly smaller(t=4.99, p<0.001)than in MI. Respiratory rate increased significantly in both MI(46.5 ± 30.9%)and MA(27.7 ± 14.6%), with no significant difference between MI and MA(t=1.82)due to the large individual variation in MI. The frequency of respiration was fairly regular during MA, but quite irregular during MI(similar to those during actual motor actions). The central nervous system which acts in MI may possess the function of activation of target effectors which play an important role in actual exercise, on the basis of incremental vigilance level induced by the function of motor anticipation.