The ultrastructure of the blood vessels of the endometrium was analysed during implantation of the blastocyst in rats, at the time of appearance of the Pontamine Blue Reaction. Vessels from implantation sites and from interimplantation sites were compared. In vessels from implantation sites the endothelial cells showed fenestrations covered by diaphragms. In addition, small interruptions (gaps) between the endothelial cells were observed. These features were present in vessels larger than 5 micrometers in diameter and more than 100 micrometers away from the uterine epithelium, both in the mesometrial and antimesometrial wall of the endometrium. Vessels from interimplantation sites displayed neither fenestrations nor interruptions. The endothelial cells of the implantation sites displayed morphological signs of metabolic activation. These consisted of increased numbers of polyribosomes, well developed Golgi complexes and prominent nucleoli. The fenestrations and gaps in the vessel wall were interpreted as constituting the morphological basis for the increase in vascular permeability and the consequent edema which characterize the Pontamine Blue Reaction.