The uterine receptivity for blastocyst implantation in the rodents is activated by nidatory estradiol and is maintained for a restricted period. To establish the variation of the RNA synthesis related to the activation of the uterine receptivity for blastocyst implantation, 1 hr. pulse 3H-uridine incorporation in the mouse uterus at time 0, 3, 6, 12 and 18 hrs. after nidatory estradiol treatment was analyzed by light (LM-RAG) and electron microscopic (EM-RAG) radioautography. The qualitative and quantitative differences of 3H-uridine incorporation in the luminal epithelium and endometrial stroma of three regions of the uterus were evaluated. In the LM-RAM, both the luminal epithelium and stromal cells around the conceptus showed a high incorporation 6 hrs. after estrogen treatment. After the peak of incorporation, the RNA synthesis both of the epithelial and stromal cells of antimesometrial side of the implantation site (AI) decreased abruptly and the grain count values become lower than seen in the other two sites. Both the epithelial and stromal cells of the IN sites showed a gradual but low increase of 3H-uridine incorporation through the time analyzed. The cells of the MI sites also showed a peak of incorporation 6 hrs. after estradiol treatment, but less intense than the AI sites. The EM-RAM showed the silver grains distributed over the cell structures related to the mRNA and rRNA metabolism both on the epithelial and stromal cells. The labeling were also seen on the trophoblastic cells, endothelial cells, smooth muscle cells of the myometrium and mesothelial cells. These results proved the strong influences of the estrogen on the pregnant uterus and showed the topological differences on the responses of the endometrium according to their relation to the implanting blastocyst, and suggest a time restricted responses of the endometrium for activation of uterine receptivity for blastocyst implantation.