To determine the role of glucocorticoids in the appearance of the facilitatory effect of stress on the ACTH response to a subsequent stress, sham-operated (Sham) rats and rats adrenalectomized (ADX) and supplemented with 50 mg/l corticosterone (B) in the drinking saline (ADX + B) were subjected to 1 min of immobilization stress (Imo) four consecutive times with an interstressor interval of 90 min. Sham rats showed a similar pattern of ACTH response to the first and fourth exposures to Imo. ADX + B rats showed an exacerbated ACTH response to the fourth Imo, despite higher prestress levels than those observed before the first Imo. In another experiment, no facilitatory effect of previous stress on ACTH response was found in ADX rats, but supplementation with B in the drinking saline for 1 wk resulted in facilitation of the ACTH response. We conclude that repeated exposure to a short-time stress induces a facilitatory effect on the ACTH response that is uncovered by eliminating stress-induced glucocorticoid release but needs B doses resulting in approximately basal circulating glucocorticoid levels to be induced or expressed.