In an earlier study, it was observed that the weight of adrenal glands of lactating rats decreased greatly when the period of lactation was prolonged for more than 20 or 25 days (Tomogane, 1969). Although Thatcher & Tucker (1968, 1970a, b) suggested that adrenal gland function may be a factor limiting milk synthesis during prolonged lactation in rats, they found no significant differences in plasma corticosteroid concentrations at the resting stage in rats between normal and prolonged lactation. Therefore, the magnitude of secretion of corticosteroid in response to the suckling stimulus was studied in rats at various stages of normal and prolonged lactation. Grosvenor and his colleagues have demonstrated that the relative importance of the suckling stimulus in inducing the release of prolactin decreased along with the advancement of lactation in rats (Grosvenor & Turner, 1958; Grosvenor, Maiweg & Mena, 1970; Mena & Grosvenor, 1972). Thus, whether a similar