New sampling protocols combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) allow the assay of single dense core vesicles. Understanding the packaging of vesicles is important as vesicles are the quanta of information for intercellular communication. Using vesicles from the exocrine atrial gland of Aplysia californica as the model, a wide range of bioactive peptides are detected within each vesicle. Although the expression of the egg-laying hormone gene family of type 1 atrial gland cells has been previously examined, chemical characterization of individual 1-2 microm diameter vesicles demonstrates that products from several genes are colocalized. The mass sensitivity of MALDI MS can be further improved to enable the analysis of even smaller subcellular organelles.