The murine retina provides an ideal model for the study of vascular development. In this investigation we have examined the development of blood vessels in flat-mounted whole retinas from C57B6 mice ranging from birth to 4 months of age. Basement membrane components of blood vessels were visualized by indirect immunofluorescence with antibodies against type IV collagen and laminin. Endothelial cells (EC) were labeled with a plant lectin, Ricinus communis agglutinin I (RCA), and antibodies against angiotensin converting enzyme. Results show three stages of vascular differentiation. During the first stage (postnatal days P0-P10), vessels develop radially from optic disc to ora serrata within the presumptive nerve fiber layer. In the second stage beginning P4, vessels form within deeper retinal layers. In the third stage beginning P7, a capillary network develops as branches of radial vessels in the nerve fiber layer. The entire vascular system begins as a polygonal network of capillary-like vessels. Selective regression of various segments of these polygons leads to the ultimate arborous pattern of arteries, arterioles, veins, venules, and capillaries seen in the adult. Some individual EC appear to be left behind during this retraction process and pericytes may have a role in determining which vessel segments regress. This combination of flat-mounted whole retinas and probes specific for vascular elements provides an ideal system for the study of retinal vascularization and the characterization of vasculogenesis in general.