By controlling access to the brain, the blood-brain barrier (BBB) restricts the entry of proteins and potential drugs to cerebral tissues. We demonstrate here the transcytosis ability of aprotinin and peptides derived from Kunitz domains using an in vitro model of the BBB and in situ brain perfusion. Aprotinin transcytosis across bovine brain capillary endothelial cell (BBCEC) monolayers is at least 10-fold greater than that of holo-transferrin. Sucrose permeability was unaffected by high concentrations of aprotinin, indicating that transcytosis of aprotinin was unrelated to changes in the BBCEC monolayer integrity. Alignment of the amino acid sequence of aprotinin with the Kunitz domains of human proteins allowed the identification and design of a family of peptides, named Angiopeps. These peptides, and in particular Angiopep-2, exhibit higher transcytosis capacity and parenchyma accumulation than aprotinin. Overall, these results suggest that these Kunitz-derived peptides could be advantageously used as a new brain delivery system for pharmacological agents that do not readily enter the brain.