A heterogenous group of mentally retarded children with I.Q. c. 50 and aged up to 18 years was compared with a group of similarly aged “bright normal” children. Two test procedures were carried out: the Critical Flicker Frequency threshold (CFF) and a maximal speed 15 sec Tapping test. Each test was repeated serially on a minute by minute basis. While significant differences between the groups were found in the maximal speed tapping test average scores, graphing the minute by minute serial test scores against time fore each individual revealed the emergence of a continuously changing series of scores which formed spontaneous cycles of recurrence. The rough graphed plots with their approximately best fit lines were confirmed by auto‐correlation tests, the statistical significance of which was found to be statisfactory. This finding applied equally to the CFF, its deviation about the means and to the tapping test when the combined scores for the right and left hands were considered and the deviation about their means. Periodicity of recurrence was found to be different between the retarded group and the group of normal children used as controls. The neurological implications of this finding are discussed.