One to 20 mg of horseradish peroxidase (HRP) was injected into the allantoic vein of chick embryos between the 7th to 21st day of incubation and its penetration from blood into the interstitium of cerebellum and spinal cord (upper thoracic) was examined by light and electron microscopy in order to study development of the blood-brain barrier. Until the 12th day of incubation (stage 38) the reaction products, indicating the presence of peroxidase, were distributed in every region of the intercellular spaces in both cerebellum and spinal cord, whereas in 13th and 14th day embryos (stage 39 and 40) they were observed only in the medullary regions. In the embryos after 15th day (stage 41) they filled only the intravascular lumen and some micropinocytotic vesicles in the endothelium. These results show that the blood-brain barrier to HRP develops roughly synchronously in various parts of the central nervous system and evolves independently of neurogenesis, the course of which differs considerably in the cerebellum and spinal cord.