Effects of differing absolute and relative doses of progesterone and estrone on implantation and fetal survival in ovariectomized rats.

Rats were inseminated, then ovariectomized and treated with different doses of progesterone (P) and estrone (E), to determine the minimal and optimal daily doses of P and E required for implantation and fetal survival in ovariectomized rats. The minimal daily dose of P and E required for an optimal rate of implantation and fetal survival in ovariectomized rats was between 0.4 and 4.0 mg of P, and 0.1 and 1.0 microgram of E. The daily 5.0 micrograms E dose was too high to realize implantation or fetal survival when it was injected with P beginning on the day after insemination. If, however, 7 days of daily P treatment preceded the daily 5.0 micrograms E, combined with 20 mg P, delayed implantation was realized but subsequent fetal survival was not. High P doses alone did not substitute for combining E with lower P doses.