In order to clarify the electrophysiological aspects of the hyperkinetic syndrome, the late component of CNV (CNV immediately before S(2) for 200 msec) and reaction time (RT) were measured in 15 patients with hyperkinetic syndrome, 15 patients with emotional disturbance, and 30 normal children, all 7-12years old. In the normal children the late component of CNV increased in amplitude, while RT was shortened, as the age increased. The late component of CNV was considered to attain a level comparable to that of adults by the age of approximately 9 years. Only in the cases of hyperkinetic syndrome was the late component of CNV significantly low in amplitude. As to RT, however, no significant difference was observed among the three groups. In the hyperkinetic children, significant correlation was observed neither between IQ and the late component of CNV, nor between the normal and abnormal EEG groups and the late component of CNV and RT. Comparing responders with nonresponders to methylphenidate, it was found that the late component of CNV of the responders was significantly lower in amplitude than that of the nonresponders, but no such difference in RT was observed. In the two cases of hyperkinetic syndrome, methylphenidate was effective in increasing the amplitude of the late component of CNV. Concerning the hyperkinetic syndrome, the present findings were discussed in relation to the functional balance of midbrain reticular formation and the thalamus, and to the possible application of the late component of CNV to the clinical fields.