THE definition of cellular organelles has evolved over the last hundred years largely driven by morphologic observations, but more recently has been supplemented and complemented by functional and biochemical studies (Palade, 1975). Thus, organelles are now identified both by their morphology and by the set ofcomponents that comprise them. Determining how organelle identity is established and maintained and how newly synthesized protein and membrane are sorted to different organelles are the central issues of organellogenesis. Essential to the many cellular functions that take place within the central vacuolar system (which consists ofthe ER, Golgi apparatus, secretory vesicles, endosomes, and lysosomes) is membrane traffic which mediates the exchange of components between different organelles. There are two critical characteristics of membrane traffic. First, only certain sets oforganelles exchange membrane and the patterns of this exchange define what are called membrane pathways. Second, multiple pathways intersect at specific points within the central vacuolar system. For specific components to" choose" the correct pathway at such points of crossing, mechanisms exist to impose choices on specific molecules. This process is called sorting. The characteristics of each organelle within the central vacuolar system are likely to be intimately tied to the properties of membrane traffic. An imbalance in the magnitude of membrane input into and egress from an organelle would have profound effects on the size ofthat compartment. In addition, failures in sorting or aberrations in targeting pathways would be expected to profoundly affect the identity of individual organelles. Recently, the relationship between the control of membrane traffic and the maintenance of organelle structure has been investigated with the use ofa remarkable drug, brefeldin A (BFA).'In this review we will summarize recent findings with BFA and propose some speculative models concerning the mechanism and regulation ofmembrane traffic within the central vacuolar system.