Changes in the activity of hypothalamic-pituitary adrenal (HPA) axis were investigated in experimental model of chronic fatigue syndrome (CFS) induced by intraperitoneal administration of synthetic double-stranded RNA (polyriboinosinic: polyribocytidylic acid, Poly I : C) to rats in the dose of 3 mg/kg body weight. In order to reveal functional changes in different links of the HPA axis, standard probes with intraperitoneal administration of ACTH and hydrocortisone against the background of cold stress application and Poly I : C injections were performed. A single injection of Poly I : C led to disordered HPA axis functions which was manifested by decreased sensitivity of the cells in the adrenal gland in response to ACTH, and suppression of the mechanism of negative feedback resulting in significant fall of cortisocterone concentration in standard assays with ACTH and hydrocortisone administration.