To investigate as to whether or not biological growth factors known to be present in natural milks could influence postnatal development of gastrointestinal (GI) tract, tests were made to determine the enteric mucosal, protein and DNA contents, alkaline phosphatase and disaccharidase activities in mongrel puppies at birth and after four days of both mother-reared and artificial reared. Microvilli of the jejunal segment were also investigated histologically by electron microscopy. Similar increases in body weight over the first four days of life were obtained and mucosal protein and DNA contents in the small intestine were greater in the mother-reared animals than in the newborn animals but neither mucosal protein content nor DNA content of the artificially reared animals was different from that of the newborn animals. Alkaline phosphatase activity was greater in both the mother-reared and artificially reared animals than that of the newborn animals. The disaccharidase activities were not different among the three groups. The jejunal microvilli of the mother-reared animals were more elaborately grown in the structure than those of the artificially reared or newborn animals. Therefore, this study demonstrated that the mother rearing over the first four days of life resulted in acceleration of the enteric mucosal growth, and the result indicates that breast feeding plays an important role in the development of the GI tract during the neonatal period.