Methyl isobutyl ketone (MIBK) is widely used in the coatings industry. A 2007 NTP study identified that MIBK induces liver tumors in B6C3F1 male (M) and female (F) mice; however, MIBK lacks genotoxicity. Previous studies suggested that the mode of action (MOA) by which MIBK induces hepatocellular tumors in mice is through phenobarbital-like nuclear receptor associated activation. To further investigate the MOA for MIBK-induced murine liver tumors, M and F B6C3F1, C57BL/6, and Car/Pxr Knockout (KO) mice were exposed to either 0 or 1800 ppm MIBK for 6 h/d, 5 d/w for a total of 10 days. On day 1, mice were implanted with osmotic mini-pumps containing 5-Bromo-2-deoxyuridine (BrdU) 1 h following exposure and humanely euthanized 1 to 3 h following the final exposure.  B6C3F1 and FC57BL/6 mice had statistically significant increases in liver weights compared to controls that corresponded with hepatocellular hypertrophy and increased mitotic figures. Hepatocellular proliferation data indicated induction of S-phase DNA synthesis in B6C3F1 and C57BL/6 mice exposed to 1800 ppm MIBK compared to control, no increase was observed in MIBK exposed Car/Pxr KO mice.  Liver gene expression changes indicated a maximally-induced Cyp2b10 (Car-associated ) transcript and a slight increase in Cyp3a11(Pxr-associated) transcript in B6C3F1 and C57BL/6 mice exposed to 1800 ppm MIBK compared to controls,  but not in Cyp1a1 (AhR associated) or Cyp4a10 (PPAR-α-associated) transcripts. Car/Pxr KO mice showed no evidence of activation of AhR, Car, Pxr or PPAR-α nuclear receptors via their associated transcripts. MIBK induced hepatic effects are consistent with a phenobarbital-like MOA where the initiating events are activation of the Car and Pxr nuclear receptors leading to rodent liver tumors.   Funding provided by the Ketones Panel of the American Chemical Council.