Adrenocortical influences on free-operant avoidance behavior.

Rats were conditioned to avoid shock on a free-operant avoidance schedule in which no exteroceptive stimulus signaled impending shock. Injections of adrenocorticotropic hormone (ACTH) or dexamethasone raised blood levels of glucocorticoids. These increases were accompanied by changes in avoidance performance: there was a higher frequency of long-duration interresponse times, a greater stability among them, and fewer short interresponse times, total responses, and shocks.