Rat pheochromocytoma (PC12) cells have been used as a model of developing neurons to study exocytosis during differentiation. Upon treatment with nerve growth factor, PC12 cells become more neuronal-like. Using amperometric detection at carbon fiber microelectrodes, time-resolved exocytosis of electroactive catecholamines can be observed. The site of exocytosis has been compared for differentiated and undifferentiated cells. Upon differentiation, cells release catecholamines primarily from varicosities along their neurites with no release from the cell body. In addition, the mean vesicular content is not significantly altered upon differentiation although it appears that the distribution of vesicle content becomes more narrow. The number of release events observed also decreases as the cells become more neuronal in character. It is possible that the smaller range of vesicle dopamine content and the decreased number of release events observed after differentiation are a result of the relocation of the site of exocytosis.