The development of the vertebrate limb requires the formation of a normal vasculature to nurture the soft and hard tissue phenotypes. The pattern of embryonic limb bud vessels has been extensively studied, but little is known about the permeability characteristics of the developing circulation. In the present study, the microvascular endothelial cell phenotype was examined by in vivo confocal microscopy following the systemic injection of a graded series of fluorescent dextrans (40,000, 70,000, 150,000 molecular weight) into chick embryos at stages 21–23 in order to determine how selective is the endothelial lining of microvessels as a partition between the blood vessels and the interstitium. Videodensitometry, over a gray scale range of 0–255, was used to quantitate the amount of tracer found within the interstitial compartment of the limb. The tracers of larger molecular weight (70,000, 150,000) were confined exclusively to the vascular lumina, whereas that of smaller molecular weight (40,000) was found to cause perivascular brightening due to extravasation into the surrounding interstitium. The reported differences in permeability were not dependent upon the stage of the embryo used in this study, but were due to the size of the tracer. These data indicate that embryonic wing microvessels demonstrate permselectivity to macromolecular efflux across the endothelium. The present results provide a basis for additional studies concerned with the dynamic characteristics of the limb microvasculature and challenge our concepts about the role of diffusible morphogens in vertebrate limb development.