Abstract: Recent studies indicate that circulating peptides or plasma proteins, such as insulin or transferrin, or modified proteins, such as cationized albumin, undergo receptor-mediated or absorptive-mediated transport through the brain capillary wall, i.e., the blood-brain barrier (BBB). Although morphologic studies such as autoradiography or immunoperoxidase labeling can demonstrate transport of blood-borne protein into brain, there is a need for a rapid, sensitive, and quantifiable physiology-based technique for comparing the relative rates of transport of several different blood-borne peptides or proteins into brain. Therefore, the present investigations describe a carotid arterial infusion technique coupled with a capillary depletion method for quantifying transport of blood-borne cationized albumin, cationized IgG, and acetylated low-density lipoprotein (LDL). Because differentiation of true transcytosis into the postcapillary compartment of brain parenchyma from binding and/or endocytosis to the brain microvasculature is important, the present studies use a dextran density centrifugation step to deplete brain homogenate of the vasculature. In addition, 3H-labeled native albumin is used as a vascular space marker to account for release of capillary contents into the postcapillary supernatant following homogenization of brain. This study demonstrates rapid transport of cationized IgG or cationized albumin into brain, as these compounds achieve a volume of distribution of 20–30 μl/g within 10 min of arterial perfusion. Conversely, acetylated LDL, although rapidly bound by cerebral microvasculature, is shown not to undergo transport into the postcapillary compartment of brain parenchyma. These studies provide the basis for a sensitive, quantifiable technique for studying transport of radiolabeled blood-borne peptides and proteins across the BBB of anesthetized animals.