Exocytosis, or the fusion of cytoplasmic vesicles with the cell membrane, occurs in nearly all eukaryotic cells, but its mechanism is not understood. Morphological and electrophysiological studies have suggested that membrane fusion begins with the formation of a 'fusion pore', a narrow channel across the closely adjacent membranes of vesicle and cell that forms the first connection of the vesicle lumen with the cell exterior and later dilates to allow release of vesicle contents. We used the patch clamp technique to study exocytosis of single giant secretory vesicles in mast cells of beige mice. The first opening of the fusion pore was found to generate a brief current transient, whose size and direction indicated an initial pore conductance of about 230 pS and a lumen-positive vesicle membrane potential. In time-resolved a.c. admittance measurements, the pore conductance was found to increase to much larger values within milliseconds, as if the pore dilated soon after opening. We conclude that the earliest fusion event may be the formation of a structure similar to an ion channel. Its conductance is of the same order of magnitude as that of a single gap junction channel, the only other known channel that spans two membranes.