This paper is a progress report on a series of experiments which have sought to probe the cognitive development of mildly mentally retarded and normal children, using conservation training as the principal research tool. Three- and 4-year-old children were given Verbal Rule instruction in a 2 x 2 x 2 design intended to probe the importance of identity, reversibility, and compensation explanations in the training of number and length concepts. The method replicated an earlier study of educationally subnormal children. Striking similarities were found in the responses of both groups of children. Identity was the primary factor in conservation acquisition; reversibility was important as well; compensation strategy proved to be of little value to either group. Justifications offered by both groups also were very similar. These varied according to the quantity being explained rather than the training received. Implications of the findings for use of conservation training as a diagnostic tool by which the truly retarded may be differentiated from other educationally subnormal children are briefly discussed. (Author/RH)