Previous reports by Kass and others have suggested that in the rabbit the nature of the adrenal secretion alters with chronic ACTH administration. A highly purified synthetic ACTH ((31,24-corticotropin) in 1 ml vehicle (peanut oil with 5% beeswax) was given im daily for 21 days to 6 male and 6 female mature rabbits. Corresponding control animals received only the vehicle. After decapitation on day 22, serum was assayed by competitive protein-binding for total corticoids, cortisol, corticosterone, cortisone, 11-desoxycortisol, and corticosteroid-binding globulin. Total androgens, thyroxine, and thyroxine-binding globulin as well as other serum constituents were also measured. Large increases in total corticoids were accounted for almost entirely by cortisol which increased 6-fold in the males and 10-fold in the females. A significant increase in total androgens was also noted while serum glucose and serum cholesterol levels fell. This study confirms that there is a radical alteration in the nature of the adrenocortical secretion of the rabbit with chronic ACTH stimulation and therefore presumably with chronic stress. {Endocrinology 91: 607, 1972) *T*HE PRINCIPLE secretory products of the -*• adrenal cortex of adult mammals are corticosterone and cortisol. Bush (1) showed that the relative quantities of these compounds vary from species to species. He and other workers have demonstrated that the rabbit is predominantly a corticosterone-producer, the corticosterone/cortisol ratio being approximately 20:1. Based on the observation that administration of corticosterone to rabbits had no effect on the lymphoid tissue whereas the administration of ACTH had a pronounced effect, Kass et al. (2) investigated the secretion of the adrenal cortex following a 7-28-day period of porcine ACTH administration. This treatment converted the rabbit into a predominantly cortisol producer. Due to the insensitivity of the methods then available only single observations on the pooled adrenal vein blood of 4-6 rabbits were made. In a review by Saba et al. (3), ACTH is said to have two effects on adrenal secretion. The first effect is seen as an immediate rise in the output of the adrenal steroids without a change in their ratios. The second effect, after prolonged ACTH administration, manifests itself Received February 9, 1972. * Presented in part to the 14th Annual Meeting of the Canadian Federation of Biological Societies, Toronto, June 1971. f Present address: School of Veterinary Medicine, University of Pennsylvania. % Associate, Medical Research Council of Canada. as an alteration in the ratio of the adrenal products. The present study was undertaken to confirm and extend the evidence that prolonged ACTH administration changes the ratio of steroid output by the adrenal cortex in vivo. Materials and Methods Mature male and female New Zealand white rabbits weighing 2.7-3.5 kg were maintained on Purina rabbit pellets and water ad libitum. A regimen of 14 hr of light and 10 hr of darkness was maintained throughout the study. A noise generator consisting of a fan to which paper strips and a balloon were attached was placed in the room to buffer the disturbance of people coming in and out, banging of doors, etc. The ACTH used was the purified synthetic Cortrosyn (Pl,24-corticotropin) which was kindly donated by the Organon Co., Montreal. Sephadex G-25 and LH-20 were obtained from the Pharmacia Co., Uppsala, Sweden. Radioactive steroids were purchased from the New England Nuclear Corp., 575 Albany St., Boston, Mass. 02118, and were used without further purification. Specific activities ranged from 10-40 Ci/mM. Non-radioactive steroids were obtained from the Sigma Chemical Co., 3500 DeKalb St., St. Louis 18, Mo. All organic solvents used were redistilled once. The competitive protein-binding methodology of Murphy (4) was used for the duplicate determination of total corticoids (TC) using dog CBG (5), total androgens (TA) (6) and thyroxine (T4) (7) as described previously. The total corticoids were subjected to fractionation by Sephadex LH-20 column chromatography (8,9) and the individual corti-