The PC12 pheochromocytoma line is a clonal line derived from a rat adrenal medullary tumor. PC12 cells grown in vitro have morphologic and cytochemical features in common with normal chromaffin cells in varying stages of development, and with human pheochromocytomas. These features include catecholamine stores demonstrable by formaldehyde-induced fluorescence and argentaffinic secretory granules, measuring 30 to 350 nm. Dark "norepinephrine"- and light "epinephrine"-type granules are both present, despite the absence of epinephrine and of its synthesizing enzyme, phenylethanolamine N-methyltransferase. Addition of nerve growth factor to the culture medium causes the cells to stop dividing and to develop neurite-like processes. Nerve growth factor-treated cells also develop clusters of 30- to 120-nm. granules and of 30- to 70-nm. granular and agranular vesicles, which resemble the granules and vesicles in adrenergic and cholinergic neurons and in neuroblastomas. In the early stages of process formation, formaldehyde-induced fluorescence can be demonstrated both in cell bodies and in processes. In later stages there is a marked diminution of formaldehyde-induced fluorescence in cell bodies and processes and a decreased number of granules in cell bodies, except in occasional cells within large clumps. These residual, fluorescent, granule-containing cells also remain argentaffinic. Alterations of the cell surface and of cytoplasmic filament arrangements also occur in cells treated with nerve growth factor. Further studies of the PC12 line may help to clarify relationships between morphology and function in the developing and mature autonomic nervous system, and the influence of nerve growth factor on these relationships.