Two aspects of the physiology of the developing gastrointestinal tract are discussed. The first one deals with functional peculiarities of the developing gastrointestinal tract. Examples are shown that less active function in the suckling (eg, low pepsin digestion in the newborn stomach) may actually give the suckling an advantage for another process. The existence of pH close to neutrality in the suckling's stomach enables "survival" of the milk and lingual lipases that are then "cooperating" with low pancreatic lipase known to be present in the sucklings. The other aspect discussed deals with the role of various factors involved in postnatal maturation of the gastrointestinal function. Using sucrase activity as an example, the important role and mutual interaction of fetal (genetic?) programming, dietary changes, and hormonal maturation are reviewed.