Thrombin induces a variety of functional alterations in endothelial cells (EC), including the production of biologically active molecules. An example is synthesis of platelet-activating factor (PAF), a phospholipid with diverse biologic actions. Thrombin was the first naturally occurring agonist identified that induces synthesis of PAF by human endothelium. 1, 2 The mechanisms involved are of considerable interest because induction of PAF synthesis by thrombin requires occupancy of a surface receptor on the EC, indicating that signal transduction and other regulatory events are involved and that thrombin may be a prototype agonist for studies of biochemical control in endothelium. The issue is also important because of the potential biologic and pathologic significance of PAF synthesis by EC. PAF exerts its biologic effects by binding to specific receptors on target cells, such as platelets and neutrophils (PMNs). 3 Its structure, l-0-alkyl-2-acetyl-3-glycero-3-phosphocholine, 3 has several features required for recognition by the PAF receptor and optimal biologic activity. PAF is released into the fluid phase by some cell types that synthesize it, such as activated monocytes, 4, 5 but it is retained by others. This was first clearly shown in endothelial cells1, 6 and has been confirmed in subsequent reports. 7, 8 In EC, a part of the newly synthesized PAF is translocated to the plasma membrane. 9 PAF can function as a mediator of juxtacrine intercellular interactions when associated with the plasma membranes of activated cells, 9 as well as when in the fluid phase (see later). Thus, the retention of PAF by EC may be a mechanism that localizes and regulates its biologic activity. There are two other important mechanisms that regulate the biologic effects of PAF. The first is that plasma, and many cell types, contain one of a family of Ca2+-independent phospholipases with specificity for substrates with short acyl chains at sn-2 that degrade PAF (PAF acetylhydrolases). 10 The plasma form of PAF acetylhydrolase limits the half-life of the molecule to a few minutes in the blood of normal subjects. 11 The second regulatory mechanism that influences the biologic effects of PAF is that its synthesis is tightly controlled. The cultured EC has been a particularly informative system for dissecting the specific events that regulate its production. 12, 13