Colostrum and milk contain insulin, epidermal growth factor and prostaglandins. Experiments performed in our laboratory have demonstrated that these three substances can be absorbed by the gastrointestinal tract of suckling rats in biologically active form in considerable amounts. Prostaglandins are metabolized by the gastrointestinal tract during postnatal development. There are quantitative and qualitative differences in the metabolism of orally administered prostaglandins between suckling and weanling rats. Nevertheless, a significant amount of intact prostaglandin appears in the liver of suckling rats after oral administration. Gastric administered insulin can result in hypoglycemia. In suckling rats, there is a considerable decrease in blood glucose levels, while no effect is seen in 30-day-old rats. Epidermal growth factor is processed in the gastrointestinal tract as shown by size exclusion and affinity columns and surface receptor binding. Oral administration of epidermal growth factor has several effects on gastrointestinal functions. Analysis of epidermal growth factor processing by the gastrointestinal tract demonstrated postnatal developmental differences (either during normal development, or one accelerated by steroid treatment). These studies show that the gastrointestinal tract can be influenced by orally administered hormones known to be present in milk, and that these substances can be delivered to peripheral organs of suckling animals intact.