The finer vascular system of the extra-placental uterine mucosa of the rabbit during oestrus and the early stages of pregnancy and pseudopregnancy was studied by means of the benzidine-nitroprusside blood stain and by injection. In the oestrous uterus there is a marked mesometrial hyperaemia which intensifies during the early stages of pregnancy and pseudopregnancy. No spiral arteries nor arterio-venous anastomoses were found in the uterine endometrium. During the early stages of pregnancy, as the uterine mucosa proliferates, there is an intense growth of new blood vessels, accompanied by a corresponding increase in the amount of blood in the vessels. This occurs throughout the uterus at first, but after 8 days post coitum the increase continues around the conceptus but slows down and finally stops between the conceptuses and in the pseudopregnant horn. When the trophoblast of the blastocyst wall fuses with the uterine epithelium the fusion areas become well vascularized. It is suggested that this vascularization is stimulated by the invading trophoblast. The trophoblast actually breaks down the walls of the maternal capillaries and allows the maternal blood to bathe the embryonic syncytium and during the time that this is happening there is a marked leucocytosis around the fusion areas. The results are discussed in relation to the previous literature and to their possible physiological significance.