The signaling lipid sphingosine-1-phosphate (S1P) plays critical roles in mammalian biology. The concentration of S1P is high in blood, and plasma S1P stabilizes junctions between vascular endothelial cells. The concentration of S1P is low in lymphoid tissues compared to blood and lymph, and S1P compartmentalization is required to maintain proper lymphocyte circulation. Although S1P in lymphoid organs is low in homeostasis, the concentration of S1P may increase upon inflammation, and increases in tissue S1P have been reported to promote angiogenesis and to enhance pro-inflammatory responses of innate and adaptive immune cells. Drugs targeting S1P signaling and S1P metabolism are currently in clinical trials as immune suppressants. By blocking lymphocyte exit from lymphoid organs, these drugs prevent activated T cells from reaching transplanted organs or organs that are subject to autoimmune attack. These drugs may also have direct anti-inflammatory effects. Despite S1P's vital functions, little is understood about how its distribution is controlled. To maintain low tissue S1P, two sources must be contained. First, tissues are constantly bathed with plasma to bring nourishment and remove waste. Plasma S1P must be prevented from entering, or removed from, the organs. Second, all cells are thought to make S1P intracellularly during the course of membrane sphingolipid metabolism. This intracellular S1P must be destroyed before it is secreted into the interstitial space. This grant investigates how S1P concentrations in the lymphoid organs are regulated.