The seasonal rhythm in hypothalamo-hypophyseal-adrenal function was studied in 3-week-old, meat-hybrid chickens, bred under standard conditions, CRF content in the median eminence and ACTH content in the adenohypophysis showed the maximum in February, the minimum in August, to return practically to the February level by November. Adrenal corticosterone production was maximum in February decreased by May and stayed at the low level for the further months of the year. The increase in autumn of the CRF content in the median eminence and of the ACTH content in the adenohypophysis, with unaltered corticosterone production suggests a reduced functioning of the mechanisms regulating hypothalamic release. It is assumed that the winter maximum in the activity of the corticotropic system is due to the endogenous annual rhythm.