The release of prostaglandins and progestin into the utero-ovarian venous blood of guinea-pigs during the oestrous cycle and following oestrogen treatment.

1. 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 F(2alpha) and E(2) was determined by bio-assay and gas chromatography-mass spectrometry, and of progestin by competitive protein binding assay.2. The concentration of prostaglandin F(2alpha) on day 3 was 14.1 ng/ml.; 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(2alpha) was increased in animals pre-treated with oestrogen but was below threshold levels in similarly treated hysterectomized females.4. Prostaglandin E(2) 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 ng/ml. 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 15.6. This work establishes that prostaglandin F(2alpha) is released into the utero-ovarian venous blood, and that its concentration fluctuates in the course of the oestrous cycle and can be related to changes in luteal function.