Two studies explored the possibility of inducing conservation understanding in intellectually normal preschoolers and mildly mentally retarded children of comparable mental age. Retarded children, MA 3-10 to 5-0, received Verbal Rule instruction. Three- and 4-year-old children were given Verbal Rule training in a 2 x 2 x 2 factorial design intended to probe the importance of identity, reversibility, and compensation explanations in training number and length concepts, replicating an earlier study of retarded children (Field 1977). Results showed that both groups made considerable progress. Experiment I shows that many 4-year-olds can conserve, even when measured by Piaget's strictest criteria. Each group performed very similarly to older retarded and nonretarded children, except that the younger groups were not able, on average, to conserve as many quantities. It was found that the effects of MA and CA are different for immediate and the long-term posttests. Four-year-old children were able to generalize to untrained quantities far better than 3-year-olds, however, and in a follow-up posttest given 2 /12 to 5 months later, those children who had generalized, maintained or increased their conservation understanding while almost all 3-year-olds reverted to nonconservation. The studies reported extend downward about one year the age at which stable conservation can be achieved. (Author/RH)