The transport of circulating nutrients (glucose, amino acids, ketone bodies, choline, and purines) through the brain endothelial wall, i.e., the blood-brain barrier (BBB), is an important regulatory step in several substrate-limited pathways of brain metabolism. The in vivo kinetics of nutrient transport has been well characterized in the rat, and the kinetic constants of saturable (Km, Vmax) and nonsaturable (KD) transport through the BBB are now known for more than 30 circulating nutrients. The kinetic constants can be used to gain insight into the important rate-limiting role played by BBB nutrient transport in the regulation of brain metabolism and function. Unlike most nutrients, steroid and thyroid hormones circulate tightly bound to plasma proteins. However, owing to favorable kinetic relationships among brain capillary transit times and rates of hormone dissociation from plasma proteins and hormone diffusion through the brain endothelia, the BBB is able to strip hormones off circulating plasma proteins. With regard to peptide hormone, no specific BBB transport systems for peptides have been identified thus far. However, peptides are able to rapidly distribute into brain interstitial space at the circumventricular organs. In addition, specific receptors for insulin are located on the BBB. The presence of BBB peptide receptors provides a mechanism by which circulating peptides may rapidly influence brain function without the peptide crossing the BBB.