SUMMARY1. Blood was collected from the utero-ovarian vein of guinea-pigs on days 3, 9, 10, 11, 12, 13, 14 and 15 of the oestrous cycle, and from animals injected with oestrogen over days 4-6 and used on day 7. The content of prostaglandin F2, and E2 was determined by bio-assay and gas chromatography-mass spectrometry, and ofprogestin by competitive protein binding assay.2. The concentration of prostaglandin F2 on day 3 was 14 1 ngfml.; it fell below detectable limits on day 7, but after day 11 the concentration rose to between 15-7-17-2 ng/ml. A value of 60-7 ng/ml. was recorded on day 15. 3. The release of prostaglandin F. was increased in animals pre-treated with oestrogen but was below threshold levels in similarly treated hysterectomized females.4. Prostaglandin E2 was not detectable in blood collected on days 3, 9, 10, 11 and 12. From a threshold value of 1 1 ng/ml. on day 13, the concentration rose to 5-4 ngfml. on day 14 and to 54 9 ng/ml. on day 15.5. The progestin concentration rose from 66 ng/ml. on day 3 to values of 80-180 ng/ml. between days 9-14. It fell to 45 ng/ml. between days 14 and 15.

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