Nerve growth factor- (NGF) induced potent activation of caspase-8, and caspase-9 in PC12 cells in a time-dependent manner. Peptide inhibition of caspase activity led to a potent reduction in neurite outgrowth in NGF-treated cells. Although NGF treatment resulted in cell-cycle G0/G1 arrest as detected by FACS cell-cycle analysis, caspase inhibitors (caspase-8 and caspase-9 inhibitors) could block the G0/G1 arrest in PC12 cells. We demonstrated the unique caspase cascade, caspase-8, caspase-3, and caspase-9 to occur, in this order. In conclusion, present results confirm a unique role for the caspase-8 and caspase-9 mediated signal cascade in the differentiation of PC12 cells.