Adrenal chromaffin cells are typical paraneurons. They are of neural crest origin, secrete catecholamines and produce action potentials. In addition to adrenaline-storing (A) cells and noradrenaline-storing (NA) cells, a third calss of chromaffin cells is distinguishable. They are characterized by the smallness of the secretory granules, and hence are called small-granule chromaffin (SGC) cells. The SGC cells have two or more, long, axon-like processes, receive a heavy innervation, and contain small synaptic-like vesicles in addition to the dense-cored secretory granules of neurosecretory type. Hence, they are regarded as an intermediate form between both A and NA cells of endocrine function and sympathetic nerve cells. Adrenal chromaffin cells share the basic process of the formation, intracellular transport, and storage of the secretory granules with all the paraneurons. Thus, the secretory granules are formed in the Golgi area and contain mainly peptides. They then incorporate adenine nucleotides and catecholamines and when mature are released by exocytosis. By means of autoradiography, quantitative variations were demonstrated with regard to dopa-, dopamine- and noradrenaline-handling capacities within chromaffin cells of a single class. A similar functional differentiation may possibly be found in paraneurons other than the adrenal chromaffin cells. Hypophysectomy in the mouse suppessed the uptake of 3H-dopamine-derived radio-activity in the chromaffin cells. The SGC cells seemed to increase their number in the hypophysectomized mouse.