The subcellular distribution of two molluscan neuropeptides, the small cardioactive peptides A and B (SCPA and SCPB), has been determined in two identified Aplysia buccal ganglion neurons, B1 and B2. These neurons were previously shown to synthesize and release these neuropeptides. B1 and B2, identified by their size and location within the ganglion, were labeled by intrasomatic injection of an electron-dense particulate marker (ferritin or Imposil) permitting the unequivocal identification of their somata and proximal processes in thin sections. The somatic cytoplasm of both neurons had a conspicuous population of large dense-core vesicles along with a smaller number of compound vesicles and small lucent vesicles. All three vesicle types are found in the neurites within the neuropil and proximal axon in the esophageal nerve. Immunoreactivity was localized on the surface of thin sections by the indirect immunogold method. The primary antiserum was shown to recognize both SCPA and SCPB after the neuropeptides had been immobilized on protein-coated nitrocellulose membranes by means of glutaraldehyde, the primary fixative used to immobilize SCPA and SCPB in situ. SCP immunoreactivity was present in the lumens of the dense-core vesicles distributed throughout the cytoplasm of B1 and B2 and in dense-core regions of the Golgi apparatus in the somatic cytoplasm. Taken together with biochemical evidence that B1 and B2 synthesize and release SCPs, these data suggest that the neuropeptides are sequestered into the protein secretory pathway of B1 and B2, a distribution that supports the notion that the SCPs function physiologically as neurotransmitters or neuromodulators.