Simple and complex learning and problem solving situations were employed with mentally retarded children and adults. In the Rotation-Discrimination Complex and the Size-Discrimination Simple experiments, upper and lower test achievement subgroups were subjected to two basic types of visual discrimination at different levels of task complexity. Significant improvement in differentiation by the posterior intrinsic system was achieved by training retardates with a mental age of at least 5 1/2 years. Both groups solved a simple problem but with no improvement in differentiation. The function of intention was inefficient on both problems for both groups. On the Distance-Discrimination Complex boys did significantly worse than girls at the more difficult position. Distance perception was maturational and related to mental rather than chronological age. Incorrect responses were made with reference to a rigid, orderly space of parallels and perpendiculars. On the Hypothesis-Discrimination Complex the subjects were confronted with experimental situations in which they could choose either of two equally valid hypotheses, and when a choice was made, conditions were changed to bring about a switch to the other. Difficulty in shifting was associated with older chronological age. (Author/SN)