The levels of free oestrogen (oestrone and oestradiol-17\g=b\) 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-17\g=b\ in plasma from the active ovary rose from 25\m=.\3 ng/100 ml at \m=-\48h to a peak of 91\m=.\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\m=.\0 ng/100 ml) at \m=-\48h; it fell to 2\m=.\0 ng/100 ml between \m=-\36and \m=-\24h and then rose again to 9\m=.\4ng/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-17\g=b\ and of oestrone produced from both ovaries at an oestrous period were estimated to average 9\m=.\7and 2\ m=. \ 4\ g=m\ g.In treated ewes, a similar pattern of production of oestradiol-17\g=b\ was shown by the ewes treated with 30 mg Cronolone. That of ewes treated with 10 mg differed (P < 0\m=.\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-17\g=b\) was less (10 mg Cronolone, 8\m=.\6 \ g = m\ g ; 30 mg Cronolone, 12\m=.\1 \g=m\g; normal oestrus, 12\m=.\1 \g=m\g), and there was no biphasic production.