SummaryEffects of intrauterine growth retardation (IUGR) on the gastrointestinal tract and the pancreas were examined using a model of piglets with naturally occurring IUGR. IUGR was associated with significant reductions in gastrointestinal and pancreatic tissue weights. The reduction in tissue weight in the gastrointestinal tract was due to lowered cell numbers and was proportionate to the body as a whole. In contrast to the gastrointestinal tract, the reduction in the pancreas was due to both lowered cell numbers and smaller cell size and was disproportionately greater than the body as a whole. IUGR was associated with precocious occurrence of maltase and sucrase activities in the small-intestinal mucosa. The exocrine cells of the pancreas in the animals with IUGR appeared immature when compared with those in normal piglets. Further studies are required to examine the postnatal effects of IUGR on the gastrointestinal tract, the pancreas, and somatic development.