Calcium-dependent exocytosis of large dense core vesicles (LDCVs) comprises several steps: docking of the vesicles to the plasma membrane (PM), priming to render the vesicles release-competent and fusion with the PM. Total internal reflection fluorescence microscopy (TIRFM) enables the real-time visualization of LDCVs near the PM as they undergo changes from one functional state to the other (Nofal et al., 2007). We used this technique in combination with patch-clamp electrophysiology to study the secretion of LDCVs in bovine chromaffin cells (Becherer et al., 2007) stimulation of the cells for 5 minutes with 6 µM [Ca2+]i induced maximal secretion and a large reduction of the LDCVs density at the plasma membrane. However, 14.1 ± 1.5% of the LDCVs were visible at the plasma membrane throughout experiments, indicating they were permanently docked. We defined these vesicles as dead end vesicles. Overexpression of Munc18 2 or SNAP-25 reduced the pool size of these vesicles. Conversely, expressing open-Syntaxin increased this pool. These results indicate that the unproductive target Soluble NSF Attachment Protein Receptor acceptor complex composed of 2:1 syntaxin-SNAP-25 exists in vivo. More importantly, they define a novel function for this acceptor complex in mediating dead end docking.