In recent years, Knowledge about the biochemical nature of proteins involved in the synaptic exocytosis has rapidly increased, and SNARE-hypothesis, which assumes necessity of ATP hydrolysis by NSF protein before membrane fusion, has been proposed to explain sequential processes of exocytosis. However, some recent experimental data do not support the SNARE-hypothesis. New model (the zipper model) does not assume ATP hydrolysis before membrane fusion, rather it assume membrane fusion elicited by protein-protein interaction among syntaxin, SNAP-25 and VAMP. However, there remain many problems to be solved.