Sex difference in the circadian periodicity of thehypothalamic content of corticotropin-releasing factor (CRF) was examined in the rat. The CRF content in the female hypothalamus was higher in the morning than in the afternoon. In contrast, in the male rat under similar experimental conditions, the CRF content was higher in the afternoon than in the morning. In the female rat, a peak value of CRF content was found at 8 a.m. and a precipitous fall ensued around noon, with a concomitant sharp rise of plasma cortico-sterone levels. Ovariectomy reduced the morning levels without raising the afternoon levels, the peak value occurring at noon. Cervical ganglionectomy did not essentially affect the basic pattern of the circadian rhythm of CRF content in the female rat. These results indicate that there is a marked sex difference in the manifestation of circadian rhythm of hypothalamic CRF content, that is, the pattern of circadian rhythm in the female rat is the reverse of that found in the male rat. The rapid fall of hypothalamic CRF content around noon in the female rat always accompanies a sharp rise of the plasma corticosterone; thus, the whole pattern of changes of CRF content during a 24 h period is a mirror image of that of plasma corticosterone.