The correlation between the daily changes of the hypothalamic CRF content and the stages of the estrous cycle was examined in the rat. During proestrus and estrus, the CRF content was significantly higher in the morning (9 a.m.) than in the afternoon (4 p.m.). During diestrus, however, no essential difference was observed. In the morning, the hypothalamic CRF content tended to be higher during proestrus than during diestrus II, but in the afternoon the content was lower during proestrus and estrus, even though the difference did not attain significance. These results, indicating a correlation of daily CRF rhythm with stages of the estrous cycle, are essentially consistent with our previous finding of a sex difference of circadian rhythm of CRF activity in the rat.