Background There have been no detailed reports of the three-dimensional structure and the relationship between the extal and intal vascularizations observed successively for a long duration in the rat fetus, although many authors have studied the vascular morphology of the developing brain. This study examined the three-dimensional structure of both the extal and intal vascularizations of the prenatal ret telencephalon from embryonic days 12 (E12) to 20 (E20). Method A microvascular casting method for scanning electron microscopy (SEM) was used in this study, along with vascular staining using gold-gelatine solution-autometallography (GGS-AMG) after intravascular injection of colloidal gold, as well as hematoxylin-eosin (HE) staining for paraffin embedded specimens. Results In GGS-AMG stains, E16 fetuses had a few short perforating cortical blood vessels (SPCVs); E17 fetuses had long perforating cortico-medullary vessels (LPCVs). Older fetuses had specific patts of vascular networks in the cortex and the deeper subcortical part of the telencephalon. In the cortex, fine longitudinal blood vessels were connected by transverse channels. The deep telencephalon had fine blood vessels running in all directions. Using SEM, the extal vascularization was already visible in E12 fetuses as arborizations of arterial branches, forming a mesh of fine vascular networks covering the telencephalon. A coralliform fine venous plexus was observed in the extal vascularization of E16 fetuses. There were ring-like anastomoses and bud-like protrusions in the network of small blood vessels, most likely the angiogenesis of fetal vessels. From E12 to E16, an immature and incomplete intal vascularization began to appear. There were short blood vessels with ballooned terminals branching from the extal vascularization. They penetrated the brain tissue to form networks in the superficial layer, comparable to SPCVs. In E17 to E20 fetuses, tortuous venous branches, straight arterial blood vessels, and a fine network of small blood vessels formed the extal vascularization. There were fewer arterial than venous branches connecting to the fine networks of small blood vessels. LPCVs were noted at E17, at the time the white matter emerged. They branched from the extal vascularization, and perpendicularly penetrated the brain surface, traversing the cortical plate, and entering into the deep brain. At E17, arterial and venous blood vessels could be clearly distinguished in the extal vascularization. At E20, the cortex and white matter contained specific arrangements of networks of fine blood vessels, as seen by GGS-AMG staining. Conclusion These findings show that the development of both the extal and intal vascularization follows the development of the telencephalon. In particular, the emergence of the cortical plate and white matter on E16 and E17 influence the development of both the intal and the extal vascularization. The laminal arrangement of blood vessels was not observed corresponding to the respective laminal neuronal layers.