Corticosteroid production, ACTH content in the adenohypophysis and CRF content in the median eminence were studied in vitro and in vivo in the 3-weeks-old chicken according to SAFFRAN and SCHALLY in four different parts of the day: at 6 and 12 a.m., and 6 and 12 p.m. The chicken adrenals in vitro produced only corticosterone in fluorimetrically measurable amounts, with the maximum during the day and the minimum in the evening and at night. Sensitivity to ACTH of the adrenals in vitro was higher during the day than in the evening or at night. After paper-chromatographic separation, steroid hormones equivalent to corticosterone and cortisol could be detected fluorimetrically in the plasma. The "cortisol" level was about 2-2.5 times higher than the corticosterone level. Plasma "cortisol" level displayed the peak at 6 a.m. and the minimum at 12 p.m., while corticosterone at 12 a.m. and 12 p.m., respectively. ACTH content in the adenohypophysis was lowest at 6 a.m. then increased gradually until 12 p.m. CRF level in the median eminence fell during the forenoon, was lowest at noon, to rise up to 6 p.m. and persist at a high level between 6 p.m. and 6 a.m. Thus, in the 3-week-old chicken the functioning of the hypothalamo-hypophyseal-adrenal system displays a circadian rhythm characteristic of animals showing diurnal activity.