This project is a study of events entering into the maturation of the small intestine. The animals used are suckling mice and rats, and chick embryos. Processes under study are the development of alkaline phosphatase, sucrase, malt maltase, and lactase activities; ultra-structural development of the epithelial cells, with special reference to the miirovilli; proliferation and transit of epithelial cells. Basic methods currently in use are organ culture (embryonic tissue); administration of exogenous hormones (mice); hypophysectomy, thyroidectomy, and adrenectomy, with appropriae replacement therapy (rats). Principal aim at the present tim is to elucidate the functional differentiation of the intestinal epithelium in terms of the hormonal control of the series of events that it comprises.