The levels of free oestrogen (oestrone and oestradiol-17β) in plasma in the ovarian vein were determined in three groups, each of 27 ewes, at nine intervals at about the time of oestrus. One group had a normal oestrus while the other two had been treated for 16 days with intravaginal sponges containing either 10 or 30 mg of a synthetic progestagen (Cronolone, Searle).
In untreated ewes, the mean level (corrected) of oestradiol-17β in plasma from the active ovary rose from 25·3 ng/100 ml at −48 h to a peak of 91·6 ng/100 ml at 0 h (onset of oestrus) and then fell. There was evidence of biphasic production. The mean level of oestrone was relatively high (13·0 ng/100 ml) at −48 h; it fell to 2·0 ng/100 ml between −36 and −24 h and then rose again to 9·4 ng/100 ml at + 12 h. There was no significant change, with time, in the plasma levels of either oestrogen from the non-active ovary. The total amounts of oestradiol-17β and of oestrone produced from both ovaries at an oestrous period were estimated to average 9·7 and 2·4 μg.
In treated ewes, a similar pattern of production of oestradiol-17β was shown by the ewes treated with 30 mg Cronolone. That of ewes treated with 10 mg differed (P < 0·01). Peak level was reached at an earlier stage, relative to the onset of oestrus, and it declined more rapidly, the total amount of oestrogen produced (oestrone + oestradiol-17β) was less (10 mg Cronolone, 8·6 μg; 30 mg Cronolone, 12·1 μg; normal oestrus, 12·1 μg), and there was no biphasic production.
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