Adrenocortical response to exogenous acetylcholine (Ach) was investigated, under anesthetized conditions, in intact, hypophysetomized and head X-irradiated dogs. Intravenous injection of Ach (1 mg/kg b.w.) to intact dogs resulted in marked increases in the secretion of 17-hydroxycorticosteroids (17-OHCS) by the adrenal gland. The maximum response was seen at 10 min after the injection and a return to preinjection levels tended to occur by 60 min. This effect was abolished completely by hypophysectomy. In dogs whose heads had been irradiated with 200 and 1,000 R of X-rays 1 day previously, a considerably lower response to Ach was found; 17-OHCS output at the time when the secretion had been maximum was 44-53% less than that in non-irradiated dogs.