Summary:Changes of blood vessels in the mouse so matosensory (barrel) cortex were assessed from birth (PO) to adulthood. Surface vessel anatomy and flow were ob served directly with videomicroscopy through closed cra nial windows and with intravascular fluorescent tracers. Histology was used to determine the internal capillary density. At birth, arterioles had numerous anastomoses with each other, pial capillaries formed a dense surface plexus, and pial venules and veins were relatively small and irregular. Morphological changes over the next 2 weeks included (a) fewer arteriolar anastomoses, (b) for mation and growth of venules, (c) more uniform diame ters of all types of vascular segments, (d) increase in in traparenchymal capillary length density (Lv), and (e) de creases in superficial capillary density and diameters. A simple morphological test showed that wall shear rates at arteriolar branch points were matched on average in neo nates and adults. Flow characteristics in single vessels were evaluated. In arterioles of like diameters, (a) V ma x'The growth and development of the brain and its blood vessels are intimately linked. The somatosen sory cortex of mice and rats is an attractive model for following this development because its stereo typed neuronal organization (Woolsey, 1990) and its segregated vasculature (Woolsey and Rovainen, 199 1) are absent at birth. Bar (1980) surveyed the 935(b) peak wall shear rates, and (c) peak flows were similar at all ages; (d) velocity was very high in occasional arte riovenous (A V) shunts in newborns; and (e) flow in arte riolar anastomoses was slow and variable. Although flow was heterogeneous in all types of vessel, the marked sim ilarities in newborn and adult mice of average peak ve locities and calculated wall shear rates in arterioles of the same size suggest that blood flow regulates in part the remodeling of blood vessels during development (Rovainen et aI., 1992). The rodent barrel cortex under goes major neuronal and vascular development, func tional differentiation, and remodeling during the first weeks after birth. It provides special opportunities for testing how blood vessels grow and adapt to supply the local metabolic requirements of neural modules in the brain. Key Words: Arteriole-Anastomosis-Arteriove nous shunt-Diameter-Lv-Murray's rule-Neonate Capillary-Vascular remodeling-V elocity-Wall shear stress-Whisker barrels.angiogenesis and maturation of microvessels during the growth of the occipital cortex in rats with his tology and morphometry. Bar et al. (1986) illus trated corrosion casts of surface vessels in neonatal and adult rats in developing (parietal = somatosen sory) cortex. They showed that the surface of the cerebral cortex at birth to postnatal day 7 (PO-P7) is covered by a dense capillary plexus, which they likened to the embryonic cerebrovascular plexus. Immature surface vascular patterns included arteri olar circles and collaterals, venules crossing over arterioles, and direct connections between arteri oles and venules as candid...