Summary. In hamsters, localized areas of increased uptake of Evans Blue dye, representing the first uterine sign of blastocyst implantation, had an increased concentration of PGE and their appearance on Day 4 was prevented by treatment with indomethacin, an inhibitor of PG synthesis. Indomethacin treatment did not terminate pregnancy: the proportion of animals pregnant on Days 5, 7, 10 and 16 was not affected, although fetal mortality was slightly greater in these animals. Indomethacin treatment caused a decrease in the weights of Evans Blue sites on Day 5 and implantation swellings on Day 10, and the duration of gestation was slightly increased, indicating that implantation had been delayed. This delay did not result from changes of circulating progesterone levels or uterine blood supply. The PG synthesis inhibitors, indomethacin and meclofenamic acid reduced uterine PG concentrations and prevented the appearance of Evans Blue sites in ovariectomized pregnant hamsters treated with progesterone. It is suggested that PGs may be mediators in the uterine Evans Blue response which precedes implantation in the hamster.
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