Secretory vesicles obtained from the atrial gland of the gastropod mollusk Aplysia californica were chemically analyzed individually with a combination of optical trapping, capillary electrophoresis separation, and a laser-induced fluorescence detection. With the use of optical trapping, a single Vesicle that had attoliters (10(-18) liters) of volume was introduced into the tapered inlet of a separation capillary, Once the vesicle was injected, it was lysed, and its components were fluorescently labeled with naphthalene-2,3-dicarboxaldehyde before separation, The resultant electropherograms indicated distinct variations in the contents of single vesicles.