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A sensitive and specific method is described for the determination of oestradiol-17β and oestrone in blood by gas-liquid chromatography with the use of an electron capture detector. The rate of secretion of these steroids into the ovarian venous blood of rats was determined on the day of pro-oestrus (six animals) and at various times during the first 8 days of pregnancy (240 animals). Oestradiol output (pg/ovary/30 min, mean ± S.E.M.) was low during day 1 (the day sperm were present in the morning vaginal smear) and until 11.00 h on day 2 (59 ± 13), but rose rapidly and significantly on the afternoon of day 2 (299 ± 46). From day 3 to day 8 an increased secretion rate was maintained (451 ± 26), though this was well below the level found on the day of pro-oestrus (mean 8·8 ng oestradiol and 1·2 ng oestrone/ovary/30 min). Oestradiol secretion during early pregnancy tended to be higher in the afternoon than in the morning.

Oestrone output roughly paralleled that of oestradiol, but was only about one third as high. No distinct peak in the rate of secretion of either steroid was demonstrable on day 4. The results strengthen the view that ovarian oestrogen secretion has an essential role in ovum implantation in the rat, but are incompatible with the hypothesis that the time of uterine receptivity to the blastocyst and of uterine sensitivity to atraumatic deciduoma induction is determined by a discrete discharge of oestradiol or oestrone from the ovary on the afternoon of the 4th day post coitum — the so-called 'oestrogen surge' theory.