Development of the small intestine was studied in newborn, and 4-, 8-, and 12-day-old young of rats fed diets, during gestation, containing 24 or 4 % casein as the source of protein. Newborn prenatally protein-deprived rats had shorter, narrower intestines, reduced numbers of villi per unit length, shorter villi, and reduced numbers of absorptive cells and crypt cells. These differences had disappeared by the age of 12 days in adequately fed pups that had survived to that age.