A response of the vascular network of rat cerebral hemisphere to injury inflicted neonatally and on the postnatal day 6 was examined. After the neonatal injury, no change could be recorded, however, following similar lesion made in 6-day-old rats zones of significantly increased vascular density were observed. The data indicate a significant change in the reactivity of cerebral blood vessels to injury occurring during the earliest period of postnatal development. This phenomenon is considered to be parallel to a similar developmental increase in the ability of astroglia to form a reactive gliosis.