GTP-binding proteins are involved in a wide variety of cellular functions. One of the most recent findings is the implication of many GTP-binding proteins in the pathway of protein secretion. In the eukaryotic secretory pathway, proteins have to be sorted from each other at many steps, including the signal recognition and translocation across the endoplasmic reticulum membrane and vesicular transport between membrane-bounded organelles. I propose here that GTP-binding proteins play a critical role in these sorting events. Several models of their action which is driven by GDP-GTP exchange will be presented.