An attempt was made to detect diurnal periodicity in the hypothalamic content of corticotropin-releasing factor (CRF) activity in the normal pigeon (Columba livia) employing the intrapituitary microinjection method. It was found that CRF activity in the pigeon hypothalamus exhibits a typical diurnal rhythm, with its peak value at 6 a.m. and its trough at 8 p.m. under a lighting regimen of 12-h light and 12-h dark daily cycle. Moreover, this rhythm of CRF activity was shown to be well correlated with a diurnal rhythm of plasma corticosterone level with a definite phase shift. These findings support the concept that the diurnal periodicity of plasma corticosteroid level in the pigeon, originates in the cyclic phases of the releasing factor within the hypothalamus. The physiological significance of these findings is discussed with special reference to the hypothalamic regulation of adrenocorticotropin secretion in the pigeon.