FSH-estimations were carried out on anterior pituitary glands obtained from rats in four stages of the oestrous cycle: early dioestrus, late dioestrus, pro-oestrus and oestrus. Autopsy was carried out in one experimental series in the morning, in a second one in the afternoon. The combined results suggested a sudden fall in hypophyseal FSH-content about noon on the day of pro-oestrus. This suggestion was confirmed by estimating the FSH-content of anterior pituitary glands removed from pro-oestrous rats either in the morning or in the afternoon.