Some hepatocarcinogens induce cytomegaly, which reflects aberrant cell cycling and increased ploidy, from the early stages of administration to animals. To clarify the regulatory molecular mechanisms behind cell cycle aberrations related to the early stages of hepatocarcinogenesis, we performed gene expression analysis using microarrays and real-time reverse transcription polymerase chain reaction followed by immunohistochemical analysis in the livers of rats treated with the cytomegaly inducing hepatocarcinogens thioacetamide (TAA), fenbendazole, and methyleugenol, the cytomegaly non-inducing hepatocarcinogen piperonyl butoxide (PBO), or the non-carcinogenic hepatotoxicants acetaminophen and α-naphthyl isothiocyanate, for 28 days. Gene expression profiling showed that cell cycle-related genes, especially those of G(2)/M phase, were mostly upregulated after TAA treatment. Immunohistochemical analysis was performed on cell cycle proteins that were upregulated by TAA treatment and on related proteins. All hepatocarcinogens, irrespective of their cytomegaly inducing potential, increased liver cells immunoreactive for p21(Cip1), which acts on cells arrested in G(1) phase, and for Aurora B or Incenp, which is suggestive of an increase in a cell population with chromosomal instability caused by overexpression. PBO did not induce cell proliferation after 28-day treatment. Hepatocarcinogens that induced cell proliferation after 28-day treatment also caused an increase in p53(+) cells in parallel with increased apoptotic cells, as well as increased population of cells expressing M phase-related proteins nuclear Cdc2, phospho-Histone H3, and HP1α. These results suggest that hepatocarcinogens may increase cellular populations arrested in G1 phase or showing chromosomal instability after 28-day treatment. Hepatocarcinogens that induce cell cycle facilitation may cause M phase arrest accompanied by apoptosis.