The lysosphingolipid sphingosine-1-phosphate (S1P) and the structurally related lipid lysophosphatidic acid (LPA) elicit a wide spectrum of biological responses in a variety of cell types, including mitogenesis, cell-shape changes, migration and contraction. Recent studies have unveiled the existence of the G protein-coupled heptahelical receptor subfamily for the biologically active lysophospholipids, which consists of the two receptor subgroups specific for S1P and LPA, respectively. The S1P receptor subgroup comprises four members, i.e. EDG-1, EDG-3, EDG-5/AGR16 and EDG-6, with considerable amino acid similarity among them. The S1P receptor subtypes are coupled to different heterotrimeric G proteins, leading to the activation of a unique set of multiple intracellular signaling pathways. The expression of transcripts of the S1P receptor subtypes is wide-spread, except for EDG-6 which exhibits lymphoid tissue-specific expression. Plasma contains substantial concentrations of S1P as well as LPA. Activated platelets appear to be a major source of S1P and LPA in blood. In addition, accumulating evidence demonstrates that S1P and LPA are released from a variety of cell types in response to various extracellular stimuli. These observations demonstrate the existence of the novel signaling system comprising the lysosphingolipids and their cognate receptors, suggesting physiological and pathological roles.