Young calves have to deal with at least three major situations that require profound physiological and digestive adaptations: adaptation to extra-uterine life (up to the first postnatal week), maintenance at a pre-ruminant stage over a long period (3 to 5 months or more), and weaning. This paper reports results obtained on the development (growth and differentiation) of the gastrointestinal tract, and on digestive enzyme activities as well as some aspects of the regulation by gut regulatory peptides. In the newborn calf, the maturation of the small intestine depends on pregnancy duration (preterm vs. full term) and ingestion of colostrum from first milking. The function of gut enterocytes evolves along with the changes from fetal to adult enterocytes. The origin of dietary protein in pre-ruminant and weaning calves modifies SI morphology. Chymosin, elastase II and lactase are typical postnatal enzymes, whereas pepsin, ribonuclease and amylase become important especially following weaning. Nitrogen digestibility increases during the first month of life and is modified by replacement of skim milk powder with non-milk proteins. Milk formula supplementation with Nabutyrate increases pancreatic secretions and digestibility. The gastrointestinal tract development depends on gut regulatory peptides plasma and luminal concentrations. The response to exogenous peptides is in relation with their number and type of functional receptors and with the animal age. Experimental work with young ruminants is important not only for the species involved, but also for its implications to other mammalians.