The rat is immature at birth and undergoes major developmental changes at the end of the suckling period. This review deals with the maturation of ingestive behavior, gastrointestinal digestive and absorptive functions, liver metabolism, and brain structure and function. Each aspect of development is physiologically correlated with the dietary transition of weaning. However, it is unlikely that the process of weaning acts as a cue for the ontogenic changes. In contrast, there is strong evidence for an important role of both thyroxine and corticosterone as coordinators of maturational events in various organ systems.