The chorioallantoic membrane (CAM) of the chick embryo represents an in vivo model to evaluate microvascular function during the sequential phases of endothelial proliferation (angiogenesis), cytodifferentiation, and senescence. The principal focus of this study was to characterize microvascular barrier functions of the CAM endothelium during its nonproliferating, aging phase (senescence). A graded series of fluorescein isothiocyanate (FITC) dextrans served to index macromolecular selectivity of the senescent capillary endothelium. Extravasation of FITC dextrans 40 and 150 was restricted, while FITC dextran 10 progressively accumulated within the intercapillary interstitia during 15-min perfusion periods. Endothelial vesicle densities were greater in the first- and second-order pre- and first- order postcapillary endothelia (28 +/- 8/micron2) than those recorded for the capillary endothelium (8 +/- 3/micron2). Junctional cleft lengths (luminal to abluminal distances), on the other hand, were greater in the capillary endothelium (1.08 +/- 0.50 microns) than those recorded for the second-order pre- and postcapillaries (0.46 +/- 0.03 micron). Junctional cleft widths were segmentally uniform (20 nm) in the microvascular units. That permselectivity of the segmental microvascular endothelia was homogeneous is ultrastructurally consistent with the uniform junctional cleft widths rather than the heterogeneous cleft lengths and vesicle densities. The CAM serves as the principal respiratory exchange surface of the embryo. Thus, maintenance of colloid oncotic balance across the microvascular endothelium is likely critical to gaseous exchange. The retention of a selective barrier during the senescent phase of the CAM endothelium serves to support this concept.