Peptides and regulatory proteins hold great promise as therapeutic agents for the central nervous system (CNS). However, the blood–brain barrier (BBB) is a major obstacle to the delivery of these potential therapeutics to their site of action. We concentrate here on the vascular BBB, which is comprised of the capillary bed of the brain specially modified to prevent the production of a plasma ultrafiltrate. For many peptides and proteins, this physical barrier is reinforced by enzymatic activities at the BBB, CNS, and peripheral tissues, short half‐lives and large volumes of distribution in the blood, binding proteins in blood, and brain‐to‐blood efflux systems. Nevertheless, there are pathways through which substances can cross. Small, lipid soluble substances cross by the nonsaturable mechanism of transmembrane diffusion, but even water‐soluble peptides can cross to some degree. Many endogenous peptides and regulatory proteins cross the BBB by way of selective, saturable transport systems. For enzymatically resistant substances with long circulating half‐lives and small volumes of distribution, such as antibodies, erythropoietin, and enzymes, substances can enter the CNS in therapeutic amounts through the residual leak of the BBB, termed the extracellular pathways. Recent examples show that the BBB transporters for peptides and regulatory substances are modifiable. This provides both a therapeutic opportunity and the potential for disease to arise from BBB dysfunctions. In the last case, the BBB itself is a therapeutic target. © 2008 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 90: 589–594, 2008.

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