Growing evidence suggests that the proteasome may be dysfunctional in a number of neurodegenerative disorders, including Lewy body diseases. We have reported previously that application of pharmacological inhibitors of the proteasome to cultured cortical neurons leads to apoptotic death and formation of ubiquitinated cytoplasmic inclusions. A number of cell cycle regulatory proteins are known to be degraded by the proteasome. In light of the emerging role of aberrant cell-cycle activation in neuronal cell death, we have assessed the involvement of cell-cycle components in the effects induced by proteasomal inhibitors in cortical neurons. Death and mitochondrial dysfunction induced by lactacystin and other pharmacological inhibitors of the proteasome were prevented by flavopiridol, a specific inhibitor of cyclin-dependent kinases (Cdks). Molecular expression of the Cdk inhibitors p16 or p27, or of dominant-negative Cdk2, Cdk4, or Cdk6 was also protective against lactacystin-induced death. Flavopiridol blocked the induction of retinoblastoma protein (pRb) phosphorylation that occurred after lactacystin application, and expression of a mutant pRb that lacked phosphorylation sites was neuroprotective. These results suggest that in cortical neurons, proteasomal inhibition leads to a cell death pathway that is dependent on Cdk activation and pRb inactivation. Although cyclins D1 and E were sequestered within the ubiquitinated inclusions formed at late time points after lactacystin application, the formation of ubiquitinated inclusions was unaffected by Cdk inhibition. This suggests that there are parallel pathways regulating neuronal death and inclusion formation elicited by proteasomal inhibition in cortical neurons.