Ultrastructural changes in the rat endometrium under several conditions including experiments of the inhibition of implantation were studied to clarify the sequential changes of the endometrium and their endocrinologic background during ovoimplantation. The following pertinent findings were noted. 1) Normal pregnancy. In the luminal surfaces, the microvilli were short and arranged rather irregularly and lost their so-called glycocalyx on L3. The large fungus-like protrusions appeared on L4. On L5, the luminal surfaces were completely covered with small serrated cytoplasmic protrusions. They formed the interdigitation btween the apposed surfaces and the uterine lumen was disappeared when the uterus was fixed in situ by the arterial perfusion method. High ATP-ase activity was demonstrated at the site of interdigitation. In the apical cytoplasm, many apical vesicles, well developed Golgi apparatuses, and dilated cisternae of the granular endoplasmic reticulum were observed on L3. On L5, however, these apical vesicles decreased in number and the Golgi apparatuses were small. On the contrary, the semilunar and coated vesicles were numerous, and the lysosomes increased both in number and size. Acid phosphatase activity was detected in these lysosomes. In the glandular epithelium, progressive increase in the number of small granules and the amount of amorphous substance were observed in the glandular lumen from L2, to L4. But on L4, some autophagosomes had already appeared in the cytoplasm. Swollen and degenerated mitochondria as well as many vacuoles were recognized on L5. In the stromal cells, the cytoplasm was enlarged, and contained many polyribosomes, granular endoplasmic reticulum with markedly dilated cisternae, lysosomes and lipid droplets on L5. The cytoplasmic protrusions on L5 were elongated and showed various features; one of them was situated very closely to the basement lamina of the luminal epithelium, and the other surrounded the lumen. Some of these protrusions were connected with the desmosomes. High ATP-ase activity was localized in these cytoplasmic protrusions, on the surfaces of the stromal cells, and in the stroma itself. 2) Delayed implantation and estrogenic transformation. In delayed implantation, the luminal surfaces were covered with regular microvilli and several fungus-like cytoplasmic protrusions. The cytoplasm contained many apical vesicles and large Golgi apparatuses that were observed both on L3 and L4 in normal pregnancy. At 48 hrs after the injection of estrogen, the interdigitation between the apposed luminal surfaces was also established. In the glandular epithelium, a slight enlargement of the glandular lumen and the appearance of small granules similar to those observed in normal pregnancy were noted at the 4th hr of estrogen treatment. In the stromal cells, two cells apposed each other very closely at the 4th hr and formed large cell masses at the 48th hr of estrogen treatment...