Normal blood flow was measured in two regions of the ileum (distal and proximal) of normal birth weight (NBW) and low birth weight (LBW) neonatal piglets. Compensatory collateral blood flow in response to occlusion of vessels in the mesenteric vascular arcades was also measured in distal and proximal ileum of NBW and LBW neonatal piglets. Under normal control conditions the blood flow in the distal ileum of NBW piglets is reduced (40% less than proximal) and in LBW animals this reduction is greater (55% less than proximal). Both proximal and distal ileum of NBW animals but the proximal ileum only of LBW animals could mount compensatory collateral flow, whereas the distal ileum of LBW animals was unable to do so. This decompensation in LBW distal ileum may explain the predilection of NEC lesions for the distal ileum and proximal colon.