Autoregulation of cerebral blood flow (CBF) is an essential homeostatic mechanism. In contrast to the adult, neonatal CBF autoregulation operates within a narrow range of systemic blood pressures, and is pressure passive beyond this range. Nitric oxide (NO), one of the biochemical mediators involved in CBF autoregulation, is a potent vasorelaxant whose production is catalyzed by at least three distinct NO synthases (NOS). In order to understand the molecular basis of CBF in the newborn, we examined the expression and distribution of 3 NOS isoforms (bNOS, iNOS, ecNOS) in various sections of the brain and microvasculature of newborn (45 day old) and juvenile (6 week old) pigs (n=3). Western blots of equal amounts of total protein lysates probed with antibodies specific to each NOS were scanned by laser densitometer and revealed different patterns of expression among the tissues and differences between newborn and juvenile. The newborn brain cortex expressed 5fold higher levels of bNOS and 10fold lower levels of ecNOS than in the juvenile. iNOS levels were unchanged. Neonatal cerebral microvessels showed 3fold lower levels of bNOS and approximately equal levels of ecNOS in cerebral microvessels from juvenile pigs. We conclude that there is a net deficiency of these NOS isoforms in the immature brain and microvasculature of the newborn piglet and that the differences in expression leads to immature control of NO production contributing to altered neonatal CBF autoregulation.