Implantation was studied in 22-day-old female rats to determine the precise amounts of ovarian hormones needed for implantation of blastocysts. 3 mg of medroxyprogesterone was given to increase the percentage of animals with delayed implantation. .1 mcg estradiol-17beta given on Day 4 produced about 50% blastocyst implantation. 105 or .025 mcg estradiol-17beta given on Day 5 of pregnancy induced nidation in 8 out of 17 and 6 out of 8 rats, respectively. There was no significant difference between control and treated groups with regard to the number of animals with delayed blastocyst implantation and the average number of blastocysts recovered. Daily injection of progesterone helped to cause nidation. High doses of progesterone provide the ratio of progesterone to estrogen needed for implantation and also support the idea that nidation may be triggered by the rise in progesterone level in the presence of a constant amount of estrogen.