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1. The dynamics of the adrenocortical response to adrenocorticotrophic hormone (ACTH) was studied in anaesthetized, acutely hypophysectomized male rats. 2. ACTH test-signals were applied either in a jugular vein ('intact infused preparation') or in the aorta through the coeliac artery with the aorta ligated below this artery ('isolated in situ perfused preparation'). The adrenocortical responses were measured directly in samples serially taken from the left adrenal vein. 3. Tested ACTH signals were either impulses (injections of 0-05--1-2 mu. ACTH) or step functions (constant infusions of 0-025--1-6 mu. ACTH/min). 4. All responses showed a 3-6 min delay, larger delays corresponding to smaller input signals. The step responses reached steady-state level without overshoot. 5. The impulse responses of the isolated perfused and of the intact infused glands, as well as their step responses, were similar as to dynamic form. 6. It is concluded that there is an inherent delay in the responses of both the isolated perfused and of the intact infused rat adrenal gland. Further, unlike what has been reported for the canine adrenal gland, the intact rat adrenal gland does not appear to be appreciably 'faster' in its response than the isolated gland. Finally, the amplification factor from ACTH to corticosterone could be estimated for both preparations in the case of ACTH injections; a value of 1-3 X 10(6) (one ACTH molecule activates synthesis and release of 1-3 million corticosterone molecules) was found.