The study is devoted to the investigation of the mental capacity of children 6-7 years old with mental retardation in preschool educational institutions of compensating type. Under observation there were 160 children, out of them - 87 patients with mental retardation and 73 mental age-matched cases - the control group. The features of the dynamics of task performance in children with mental retardation in comparison with the cases from control group have been revealed. Indices of their mental capacity were found to be in dependence on the organization of upbringing and education space, which can be achieved only in a preschool educational institutions of compensating type. Mental retardation in children was found to be in a greater degree associated with somatogenic causes.