WAF1/Cip1/Sdi1 (p21) is the prototype of a family of proteins that inhibit cyclin-dependent kinases and regulate cell cycle progression in eukaryotic cells. In addition to normal cell cycle progression, p21 is involved in growth suppression mediated by p53 and transforming growth factor β (TGFβ), differentiation, and apoptosis. To gain insight into the possible involvement of p21 in liver cell growth, the expression and regulation of the p21 gene was evaluated in rodent models of liver regeneration and specimens of human liver diseases. Little p21 mRNA was detected in normal liver tissue. After growth stimulation in vivo by 70% partial hepatectomy (PH), the p21 transcript was upregulated in a biphasic manner, with enhanced expression during G1 phase and following S phase. The induction of p21 after PH was regulated primarily at the post-transcriptional level and was due to enhanced mRNA stability. Inhibition of protein synthesis with cycloheximide rapidly induced p21 expression, primarily by post-transcriptional stabilization of the transcript. Hepatic p21 mRNA was also induced by dietary protein deprivation in normal mice. Expression of the p21 gene after PH was similar in p53-deficient (p53 -/-) and wild-type mice, but was p53-dependent following protein deprivation. Primary hepatocytes in culture demonstrated increased p21 expression after treatment with hepatocyte growth factor, TGFβ, and activin A. p21 mRNA was upregulated in human liver diseases, suggesting a possible role in hepatic growth regulation in pathologic states. The present study demonstrates that p21 is regulated by p53-dependent and -independent pathways in the liver, and is influenced by both mitogenic and growth inhibitory stimuli.