The work proposed relates to the integration of events that enter into the maturation of the surface of the murine intestine, and of the control of these events. We are attempting to determine whether the appearance of a new phosphatase in the 2-week-old animal is due to synthesis or activation, and whether the appearance of this enzyme is necessarily related to the cessation of pinocytosis and/or the elongation of the microvilli, which are normally concurrent with the enzymic change. We are also examining the developmental patterns of sucrase and maltase in the same frame of reference. Glucocorticoids, cycloheximide, and BUdR are being used in these studies. In addition, we are endeavoring to enlarge our understanding of the endocrine control of intestinal maturation by hypophysectomy of rats in early infancy. The influence of this operation on both the architecture of the intestinal surface and its enzymic constitution is being examined.