Syntaxin 1, an essential protein in synaptic membrane fusion, contains a helical autonomously folded N‐terminal domain, a C‐terminal SNARE motif and a transmembrane region. The SNARE motif binds to synaptobrevin and SNAP‐25 to assemble the core complex, whereas almost the entire cytoplasmic sequence participates in a complex with munc18‐1, a neuronal Sec1 homolog. We now demonstrate by NMR spectroscopy that, in isolation, syntaxin adopts a ‘closed’conformation. This default conformation of syntaxin is incompatible with core complex assembly which requires an ‘open’syntaxin conformation. Using site‐directed mutagenesis, we find that disruption of the closed conformation abolishes the ability of syntaxin to bind to munc18‐1 and to inhibit secretion in PC12 cells. These results indicate that syntaxin binds to munc18‐1 in a closed conformation and suggest that this conformation represents an essential intermediate in exocytosis. Our data suggest a model whereby, during exocytosis, syntaxin undergoes a large conformational switch that mediates the transition between the syntaxin–munc18‐1 complex and the core complex.