Properly regulated exocytosis of neurotransmitters and hormones from membrane-delimited vesicles is essential for normal cellular function. At least 3 distinct steps are involved in regulated exocytosis: (1) the docking of vesicles at the appropriate plasma membrane, (2) priming of a subset of docked vesicles to become release-competent, i.e. able to release in response to a stimulus without further maturation steps, and (3) the fusion of the vesicle membrane with the plasma membrane, during which a fusion pore forms and dilates to release vesicle contents. A current working model to explain these steps in molecular terms is the SNARE hypothesis, which postulates that the vesicle membrane and the plasma membrane are brought into close proximity and triggered to fuse with each other by forming a stable SNARE core complex from the SNARE proteins on their membranes. Complexin, a small neuronal protein that binds to the assembled SNARE core complex, has been suggested to regulate neurotransmitter release by stabilizing the SNARE complex. Neurons lacking complexin show a dramatically reduced Ca2+-dependent neurotransmitter release, but how lacking complexin decreases the release and at which step of exocytosis does complexin participate are still unknown. In this study, we demonstrate that adrenal chromaffin cells express only one complexin isoform, complexin II. We show that chromaffin cells from complexin II knock-out mice exhibit markedly diminished readily releasable vesicle pools but show no change in kinetics of fusion pore dilation or morphological vesicle docking