Amperometric detection of exocytosis at single chromaffin cells has shown that the distribution of spike areas, or quantal size, is dependent on the volume and catecholamine concentration of individual secretory vesicles. The present work offers an alternate, simplified model to analyze the current spikes due to single exocytotic events. When the cube root of these spike areas is plotted as a histogram, a Gaussian distribution is obtained for chromaffin cells and also mast, pheochromocytoma, and pancreatic beta-cells. It was found that the relative SD of these distributions is similar to that for the vesicular radii, which also have a Gaussian distribution in all four cell types. In addition, this model was used to evaluate conditions where the quantal size of individual events was altered. When chromaffin cells were maintained in culture for < 6 days, spikes of approximately double the quantal size were obtained on repeated exposure to 60 mM K+. The results suggest a heterogeneous distribution of catecholamine-containing vesicles at later days in culture is responsible for this alteration.