Exocytotic events monitored in adrenal chromaffin cells by means of carbon fiber amperometry can be categorized in two groups according to the rate and extent of fusion pore expansion. The probability of fast fusion increases with intracellular calcium levels. It is also dependent on a short intermembrane distance, mediated by tight zippering of the SNARE complex. We also found that the GTPase cdc42 is needed for rapid expansion of the pore. The components of the fusion machinery are thus not the only determinants of fusion pore dilation.