Infants with malrotation of the gut easily develop midgut volvuluse If this volvulus is not treated immediately, ischemic necrosis of the small bowel may develop rapidly. The treatment of these children requires extensive small bowel resection. Infrequently, children are born with multiple jejunal or ileal atresias or with an 'apple peel' type of small bowel atresia. Both after massive small bowl resections and in severe atresias, the child is left with a very short length of bowel and great difficulties to thrive. Fortunately, with time both functional an::l morphological adaptation of the remnant small bowel occurs. In the neonatal period it is not clear whether growth of the small bowel after a massive resection is part of normal growth or ~hether it is due to, or may be enhanced by, adaptational responses. Neonates, especially in the recovery stage from intestinal surgery, often have to be fed parenterally, or later, with artificial milk formulas. It is known from experiments in adult animals, that oral feeding stimulates adaptation of small intestine after resections. In the newborn, colostrum seems to provide a special stimulus for gut growth, as has been suggested by animal studies and by experiments in vitro. Therefore, we thought it could be important to examine the effects of colostrum, of breast milk produced later in lactation, and of a commercial milk formula on normal development and on adaptation to resection of the intestine. l'le also studied the effect of the presence of food in the small bowel on development and adaptation. Some of the components of breast milk which are not present in artificial formulas are macromolecules, especially y-globulins, and viable rnacrophages. Breast milk provides passive immunity in some species and other host resistance factors, protecting the suckling in most species. Furthermore it has a regulatory effect on bacterial colonization in the bowel. Since necrotizing enterocolitis occurs with much lower frequency in breastfed human neonates, breastmilk may play a role in the prevention of this disease by maintaining the normal mucosal barrier to bacteria and harmful macromolecules. Surgery of the small bowel also influences the uptake and transport of macromolecules. In some studies increased transport was noted due to the loss of mucosal integrity. From studies on adaptation after resection it can be concluded, however, that after surgery premature cessation of transport of macromolecules may occur. Since uncertainty still exists about the effects of surgery on macromolecular transport, this was also studied.