p21WAF1 cyclin-dependent kinase inhibitor has been implicated in the control of proliferation, differentiation, and death in various cell lines. To further examine p21 regulation of the transitions between these cellular processes, an inducible p21 vector (lac operon system) was transfected into the rat pheochromocytoma (PC12) neural cell line. Induction of p21 led to permanent growth arrest, as evidenced by cell counts, FACS analysis, and thymidine incorporation. This arrest was maintained, even after removal of the inducing signal (IPTG). Northern analysis revealed that endogenous p21 mRNA increased following IPTG removal, which may be responsible for the continued growth arrest despite the decrease in ectopic p21 expression. p21 overexpression did not directly lead to a differentiated phenotype; however, differentiation in response to nerve growth factor (NGF) was greatly accelerated. To examine effects on cell death, and specifically test the hypothesis that apoptosis caused by withdrawal of trophic support results from inappropriate entry into cell cycle, serum was removed from proliferating and p21-arrested PC12 cells. The rate of apoptotic death was not affected by p21, nor was it effective in altering the extent of death following other apoptotic stimuli.