The calcium-independent nitric oxide synthase, known as NOS-II or inducible NOS, is only present in adult brain during pathologies that involve inflammatory events. We sought to establish whether NOS-II was also expressed in the course of normal brain development. NOS-II mRNA, measured by quantitative polymerase chain reaction, appeared during the perinatal period in different brain areas. It was detected on embryonic day 14, the earliest time analyzed, and then levels decayed until the first postnatal week when a second, more robust phase of expression arose to peak around postnatal day 7. Expression then declined to negligible levels in adulthood. Immunohistology revealed translation of the NOS-II protein in both embryonic and postnatal animals, localized to parenchymal blood vessels, whereas no vessel staining was detected in adults. Furthermore, NOS-II mRNA and enzymatic activities were present in isolated brain microvessels from developing animals but not from adults, and P7 vessels contained a 125 kDa protein band detected with a monoclonal antibody raised against rat NOS-II protein. These results indicate that the calcium-independent NOS mRNA and functional protein are transiently expressed in vessels throughout the brain in the course of normal development.