Three Experiments were conducted to investigate the relationship between intelligence and reaction time. In the first study, the blocking time of the alpha rhythm to photic stimulation was measured with an OFFNER Type T EEC from 11 retarded and 11 normal children matched on CA. The results of 20 trials for each subject show that the mean blocking time of the retarded children was longer than that of the normals. In the second and third studies, the simple reaction time and discriminative reaction time were measured with the reaction timers from 30 retarded and 30 normal children mathed on CA. The results show that the retarded children responded more slowly than did the normals on the discriminative reaction time, but both groups were not significantly different with respect to the simple reaction time. This study indicated particularly that the longer reaction time was obtained for the retarded children in more complex situations where they used of necessity more central integration time than the normal children.