Cultures of neurite-bearing pheochromocytoma (PC12) cells and of sympathetic neurons have been examined by time-lapse video microscopy. In the presence of nerve growth factor (NGF), the neurites of such cultures elongated and their growth cones changed geometry, via microspike and lamellipodial motion, on a time scale of minutes. Withdrawal of NGF caused process extension to cease and a progressive reduction in growth-cone area as a result of retraction of lamellipodia and microspikes. By approximately equal to 4 hr after NGF withdrawal, most neurite tips were smooth sided, devoid of conical expansions at their termini, and virtually immobile. Addition of NGF to cultures from which it had been withdrawn induced motion of microspikes and projections from the upper surface of growth cones within 2 min, while lamellipodial spreading and neurite reextension were induced after approximately equal to 20 min. For PC12 cells, these responses to replacement of NGF could not be mimicked by addition of dibutyryl cAMP (less than or equal to 2 mM) or the Ca2+ ionophore A23187 (less than or equal to 5 microM) to NGF-deprived cultures nor inhibited by the presence of EGTA (less than or equal to 2 mM) or calcium antagonists in the culture medium. Since neurite fragments formed by transection of processes of PC12 cells deprived of NGF responded to its replacement in a manner similar to intact neurites, it is concluded that the effects are focal to the neurite and growth cone and independent of the cell body. This influence of NGF on growth-cone shape and motility represents short-term local activation of this structure and has significance for control of neurite extension.