The role of the paraventricular nucleus (PVN) of the hypothalamus and the hypophysis in ensuring the corticosteroid feedback of the pituitary-adrenocortical system (PACS) in rats was characterized by administration of increasing doses of hydrocortisone. The doses 100, 300 and 1200 micrograms/100 g b.w. of hydrocortisone were administered intraperitoneally. The injection of hydrocortisone in the dose of 1200 micrograms/100 g b.w. reduces the pituitrin-induced rise of ACTH content in plasma. The reduction of pituitary reactivity to pituitrin was not observed with the injection of smaller doses of hydrocortisone. The involvement of the hypothalamus in ensuring the corticosteroid feedback was estimated by the suppression of the stress-induced PACS reaction in animals with bilateral PVN lesions. The inhibition of the stress-induced corticosteroid rise did not occur in animals with the PVN lesions if they were treated with hydrocortisone in the dose of 100 and 300 micrograms/100 g b.w. At the dose of 1200 micrograms/100 g b.w. the inhibition of the stress-induced reaction is observed but it is less pronounced as compared to the control. The conclusion is made that at small, close to physiological doses of hydrocortisone (100 and 300 micrograms/100 g b.w.) the PACS inhibition is ensured by PVN involvement into the feedback pathways. At supraphysiological doses of hydrocortisone (1200 micrograms/100 g b.w.) the inhibition of PACS reaction is performed due to not only the hypothalamic PVN but the reduction in the pituitary reactivity.