Brain microvascular endothelial cells (BMEC), limit the passage of substances, both soluble and cellular, from the blood into the brain. BMEC utilize unique features to distinguish themselves from peripheral endothelial cells, such as; 1) intercellular tight junctions that display high electrical resistance and slow paracellular transport; 2) the absence of fenestrae and a reduced level of pinocytic activity; and 3) the expression of specialized pumps that can transport compounds out of the brain via the blood-brain barrier. Similar to peripheral endothelial cells, BMEC express cell adhesion molecules on their surface that regulate the extravasation of leukocytes into the brain. Rat Brain microvascular endothelial cells are useful for studying the molecular and cellular properties of blood-brain barrier. Understanding the molecular mechanisms of blood-brain barrier regulation may help to optimize drug delivery to the CNS and elucidate new therapies for CNS diseases.