To summarize luteal function during pregnancy briefly, there are physiological processes initiated by the embryo and/or conceptus in early pregnancy that serve to prolong the life-span of the corpus luteum. Some of these processes are well defined, but others remain more obscure. The corpus luteum is maintained in a functional state throughout pregnancy (at least in those species described in this review), even though in several species progesterone production by the corpus luteum is not required after the first third of the gestational period. The cessation of secretory function by the corpus luteum of pregnancy at the end of gestation is apparently actively induced. There is evidence in some species (especially the goat) that this is due to PGF2alpha released from the uterus or placenta. It is concluded that the occurrence of luteal regression in several species of mammal can be attributed to the physiological release of PGF2alpha from both the pregnant and nonpregnant uterus.

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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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The production of prostaglandins by the uterus and the resting levels of prostaglandins in the uterus on selected days of the oestrous cycle were determined in guinea-pigs.Prostaglandin F2\g=a\ was detectable in the guinea-pig uterus in small amounts on days 13, 14 and 15 of the cycle. Prostaglandin E2 was present in even smaller amounts on days 14 and 15. The homogenized guinea-pig uterus had the ability to biosynthesize prostaglandins, from endogenous precursors, during incubation on every day of the cycle studied. Four to six times more prostaglandin F2\g=a\ than E2 was produced on any one day with the amounts of prostaglandins formed increasing towards the end of the oestrous cycle. Indomethacin inhibited the biosynthesis of prostaglandins by the guinea-pig uterus. The implications of these findings are discussed.