Regulated secretion and exocytosis require the controlled docking and fusion of secretory storage organelles with the plasma membrane. Proteins resident in secretory organelle membranes are, therefore, likely to have critical functions as a part of these processes. The exocrine pancreas with its secretory storage organelle, the zymogen granule, represents a classical model of regulated secretion and exocytosis. Until recently, however, little has been known of zymogen granule membrane protein structure and function. With the recent cloning of two zymogen granule membrane proteins, GP-2 and GP-3, and with the identification of the presence of low molecular weight guanosine triphosphate binding proteins, it is now possible to approach the role of the zymogen granule membrane proteins at the molecular level.