The time course of extrusion of the vesicular contents during exocytosis has been examined at adrenal medullary cells with carbon-fiber microelectrodes. Two electrochemical techniques were used: cyclic voltammetry and amperometry. Spikes obtained by amperometry had a faster time course than those measured by cyclic voltammetry, consistent with the different concentration profiles established by each technique. However, the experimental data obtained with both techniques were temporally broadened with respect to dispersion of an instantaneous point source by diffusion. Measurements with the electrode firmly pressed against the cell surface established that the temporal broadening is a result of a rate-limiting kinetic step associated with extrusion of the vesicular contents at the cell surface. The data do not support a rate-limiting process due to restricted efflux from a small pore. When combined with previous results, the data suggest that the rate-limiting step for chemical secretion from adrenal medullary cells during exocytosis is the dissociation of catecholamines from the vesicular matrix at the surface of the cell.