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Ovarian vein blood was collected by cannulating the ovarian vein of 37 ewes at various times during the oestrous cycle. Plasma progesterone and oestrogen concentrations, determined by chemical methods, showed marked cyclic variations during the cycle. Progesterone was detected at all stages of the cycle. Plasma concentrations were rising rapidly by the 4th day after the onset of oestrus and were maintained at levels greater than 100 µg./100 ml. plasma from the 8th to the 14th day of the cycle. They started to fall about 48 hr. before the onset of oestrus. Very low levels, of the order of 1 µg./100 ml. plasma, were maintained from 24 hr. before to 8–16 hr. after the onset of oestrus.

Oestradiol-17β made up almost all of the oestrogens measured. Oestrone accounted for less than one-eighth of the total oestrogens. Oestradiol first appeared on the 14th day of the cycle and its concentration rose rapidly during the immediate pre-oestrous period to reach peak levels of over 100 ng./100 ml. plasma 20–30 hr. before the onset of oestrus. They then rapidly declined and by 24 hr. after the onset had reached almost non-detectable levels.

The ovarian secretion rate of progesterone was calculated to be 3·5 mg./day at mid-cycle and the total secretion of oestradiol during the follicular phase was 4·9 µg. Blood flow through the cannula was not affected by either the stage of cycle at which blood was collected or by the structure (corpus luteum or Graafian follicle) in the ovary bled.