The dexamethasone sensitivity of the hypothalamo-pituitary-adrenal (HPA) axis was significantly impaired in Wistar rats exposed to inescapable shock. No difference was found between the rats with learned helplessness and intact animals. The feedback impairment seems to occur irrespectively of the hyperactivity of the HPA axis. Inescapable shock increases the plasma vasopressin level which, in its turn, may cause the dexamethasone resistance of the HPA axis.