The villous epithelial cells of the terminal part of the rat small intestine readily absorb maternal antibodies and certain other macromolecules up to the 18th day after birth. Between 18 and 21 days, however, these cells are progressively replaced by more mature cells, and the uptake of macromolecules declines to zero (Clarke & Hardy, 1969a, b). This process has been termed 'closure'. Closure can be induced at least 9 days before the normal time by the administration of deoxycorticosterone acetate or cortisone acetate (Halliday, 1959). Furthermore, bilateral adrenalectomy at 15–18 days after birth has been shown to delay the time of closure (Daniels & Hardy, 1971). These results suggest that the functional development of the adrenal cortex may determine the maturation of the small intestine with respect to its ability to absorb macromolecules. In order to investigate further the possible role of the adrenal gland in the mechanism of