Paraneurons are receptosecretory cells producing substances of neurons. Like neurons, they are characterized by membrane-bound granules and vesicles which contain bioactive (and inactive) peptides, amines or other classic messengers, adenine nucleotides and acidic carrier proteins including chromogranins. The cell-biological significance of the carrier proteins in the secretory granules and the utility of immunohistochemical detection of chromogranins as reliable markers of paraneurons are emphasized. Cells "ectopically" producing bioactive peptides and/or amines, including ANP producing muscle cells of the heart, do not contain chromogranin immunoreactivity. The boundary between the neuron and paraneuron and that of the paraneuron are unclear because the cells comprise gradational spectra from typical ones to atypical ones. It is arbitrary whether to include the mast cell in the paraneurons, although it seems reasonable to exclude it by defining a paraneuron as being epithelial in nature.