Recent studies have suggested that the morphological characteristics of secretory granules contained within endocrine cells and nerves may be determined largely by their chemical composition. The use of the immunogold staining (IGS) method, which is based on the adsorption of colloidal gold to immunoglobulins, has been used in our laboratory to demonstrate a wide range of intracellular antigens at both the light and electron microscope levels. In this study we have applied a modification of the IGS method for the simultaneous detection of two separate antigens in a single tissue section, using a variety of regionspecific antisera to different peptides. Peptide antisera were raised in rabbits or in guinea pigs and these were applied simultaneously or sequentially to grid-mounted ultrathin tissue sections. Antigenic sites were visualized at the electron microscope level using antisera raised in goats, adsorbed to gold particles of 12, 20, or 40 nm. Using this technique we have attempted to investigate the coexistence of multiple antigens in single tissue sections, in particular in single granules; the topographic distribution of molecular forms within one single granule or granule population; the heterogeneity of peptidergic neurons and also the heterogeneity of peptide content in morphologically similar granules. The double immunogold staining procedures described here have proved to be extremely effective for the simultaneous ultrastructural localization of two antigens (peptide-peptide; peptide-propeptide) on a single tissue section. The further development ofthis technique may provide useful information on neuroendocrine cell dynamics in normal and diseased states.