Changes in the ongoing EEG when repeatedly closing and opening the eyes are quantified and compared for a group of mildly retarded children and a matched control group. The most prominent changes occur for the alpha-rhythm at posterior derivations. Blocking of the alpha is faster than its restriction, but there is no group difference in this respect. Many of the amplitude changes are also quite similar for the two groups. The differences found are associated with a lowered arousal by the experimental group.