SUMMARY: It is shown that the uterus of the cyclic rat can be transformed into a 'delayed-implantation' uterus (i.e. a uterus in which blastocysts are maintained in a dormant condition) by ovariectomizing females and then injecting them with 2 mg. progesterone/day, for several days. Blastocysts transferred into the uteri of females so treated remained dormant for as long as the progesterone treatment was continued. Upon injecting 1 μg. oestrone together with 2 mg. progesterone, the blastocysts implanted and developed into normal foetuses. When blastocysts were transferred on the day of ovariectomy without previous progesterone treatment, they did not survive. Apparently ovariectomy followed by progesterone injections neutralized a uterine factor detrimental to blastocysts. The number of daily progesterone injections required to neutralize this factor depended on the day of the cycle on which ovariectomy was done. Based on the latter finding, it is postulated that the endogenous oestrogen (which fluctuates during the cycle) is the factor which renders the uterus inimical to blastocysts.