Signal-activated phospholipase-catalyzed hydrolysis of phosphatidylcholin involves three distinct enzymes: phospholipase A2 , phospholipase C (PLC), and phospholipase D (PLD) (1 ). PLC-catalyzed hydrolysis generates sn -1,2-diacylglycerol (DAG) and choline phosphate, whereas PLD stimulates the generation of phosphatidate (PA) and choline. Choline and choline phosphate are probably not messengers, although there have been some claims of a signaling role for the latter. DAG is the physiological activator of protein kinase C, whereas PA has an incompletely defined messenger function but has been demonstrated to activate a number of serine/threonine kinases and to play a role in secretion and rho-dependent actin stress-fiber formation (2 ). PA and DAG are apparently interconvertable through the action of phosphatidate phosphohydrolase and diacylglycerol kinase. However, work from this laboratory has recently demonstrated that the acyl structure of PLD-derived PA and PLC-derived DAG is distinct, the latter being polyunsaturated, whereas the former is more saturated/monounsaturated suggesting a specificity between the two signaling pathways (3 ,4 ). Indeed, the DAG generated from PLD-derived PA does not activate protein kinase C in vivo. It is thus of importance to be clear of the source of the DAG and PA when attempting to define the signaling of an agonist-stimulated cell. Polyunsaturated DAG is generally derived from phospholipase C-catalyzed phosphatidylinositol 4,5-bis -phosphate hydrolysis; however, there are examples of agonist-stimulated phospholipase C-catalyzed phosphatidylcholine hydrolysis (5 –7 ).