The precise nature of the impairment of hypothalamo-pituitary-adrenocortical (HPA) activity which follows prolonged corticosteroid treatment is not yet understood. To study this problem, hypothalamic corticotrophin releasing hormone (CRH) content, pituitary ACTH content and the functional capacity of adenohypophysial tissue in vitro were measured in rats after treatment with betamethasone. The content of CRH and ACTH in the hypothalamus and pituitary gland respectively were markedly reduced. After stopping the treatment the hormone concentrations in both structures returned to normal with the rise in the hypothalamus preceding that in the pituitary gland. Adenohypophysial tissue from betamethasone-treated rats incubated with hypothalamic extracts from control animals showed a considerable reduction in its ability to synthesize and release ACTH. However, corticotrophin release was impaired in adenohypophyses removed from untreated rats and incubated with betamethasone but synthesis was not affected. The physiological significance and the possible clinical relevance of the results are discussed.