Milk contains a significant number of substances having peptide characteristics that are known to possess biological activity. The possible physiological importance for the neonate is discussed in this review in light of their effects (epidermal growth factor, nerve growth factor, insulin, prolactin, somatostatin, thyroid-releasing hormone, thyroid-stimulating hormone, growth hormone-releasing factor, luteinizing hormone-releasing hormone, adrenocorticotrophic hormone, erythropoietin, bombesin-like peptides, calcitonin, beta-casomorphins and delta-sleep-peptides) on suckling mammals after gastrointestinal administration.