The liver is capable of undergoing a proliferative growth, known as direct hyperplasia, in which the naive liver increases in size due to stimulation with primary mitogens. To produce accurate gene expression data, housekeeping genes (HKGs) that are stably expressed need to be determined. In the present study, liver regeneration was promoted via the direct hyperplasia mode by inducing mice with 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene. Gene expression levels of nine commonly used HKGs were analyzed in the liver of different timing during the regeneration. The stability of gene expression Was assessed using two different analysis programs, geNorm and NormFinder. Using these analyses, we identified that PPIA and RPL4 showed the most stable expression regardless of the Status of the liver regeneration. In conclusion, the present Study demonstrated that the use of PPIA and RPL4 were the most optimal in providing reliable normalization of gene expression when assessing liver regeneration attributed to direct hyperplasia. (C) 2008 Elsevier Inc. All rights reserved.