Most studies of the NGF-dependent outgrowth of neurites have been performed with cultured sympathetic and sensory neurons or PC12 cells. The primary neurons have had prior exposure to NGF in vivo and they require NGF for basic survival. Although PC12 cells do not require NGF for survival, the outgrowth of neurites in such cells is a consequence of their differentiation into cells that resemble sympathetic neurons in response to NGF. Thus, the outgrowth of neurites is not necessarily a direct consequence of exposure to NGF. PC12D cells, a stable variant subcloned from native PC12 cell populations, produce neurites in a rapid transcription- and translation-independent manner upon exposure to NGF, bFGF, dbcAMP or staurosporine. The rapid sprouting of neurites occurs within minutes in local regions of PC12D cells that are exposed to NGF. Recent studies in conventional PC12 cells demonstrated the close relationship between the activation of MAP kinases (ERKs) and the outgrowth of neurites in response to various agents. Stimultaneous activation and rapid nuclear translocation of MAP kinases were also observed in PC12D cells treated with NGF or bFGF. But the activation of MAP kineases was not observed in the outgrowth of neurites induced dbcAMP or staurosporine. In this cell line, the NGF-dependent outgrowth of neurites was not blocked by the inhibition of the activation of MAP kinases by a MEK inhibitor, PD-98059. These results indicate that the activation of MAP kinases and subsequent expression of specific genes are required for the NGF-induced differentiation of PC12 cells but this pathway is not required for the NGF-dependent outgrowth of neurites. These processes have been investigated together in most studies of convetional PC12 cells. PC12D cells provide a unique experimental system for studies of the cellular mechanism of the NGF-induced sprouting and elongation of neurites, separately from the transcription-dependent differentiation of the cells.