The US EPA has completed two risk evaluations of 1,4-Dioxane (1,4-DX) carcinogenicity, an IRIS assessment in 2013 and the recently published, draft TSCA risk evaluation in 2019, where human cancer risks related to rodent liver tumors were characterized using the default non-threshold model. The available evidence for the rodent liver tumor response better aligns with a threshold-dependent, tumor promotion Mode of Action (MOA) but early key events (KEs) in this MOA have not been well defined for the mouse model. To address this, we conducted a study in female B6D2F1/Crl mice. Mice consumed drinking water containing 0, 40, 200, 600, 2,000 or 6,000 ppm 1,4-DX (targeted at 0, 10, 50, 150, 500 and 1,500 mg/kg/day, respectively) for up to 90-days. Blood levels of 1,4-DX and its major metabolite, 2-hyrdoxyethoxyacetic acid (HEAA), were determined to characterize the time course of absorption and clearance of 1,4-DX. Indices of hepatic proliferation, histopathology, mRNA analysis and BrdU incorporation were used to characterize the time course and magnitude of a proposed KE (proliferation) resulting from exposure to 1,4-DX. There was a dose and time dependent increase in blood levels of HEAA with a supralinear increase in 1,4-DX blood concentration with exposure to 6,000 ppm demonstrating saturation of metabolic clearance. There was a corresponding, and previously unreported, increase in liver weight and BrdU incorporation in the absence of cytotoxicity and genotoxicity indicating a direct mitogenic stimulus triggered by 1,4-DX accumulation. These data advance the understanding of early KEs in the MOA for 1,4-DX and confirm previous reports of a threshold MOA based on metabolic saturation and accumulation of parent compound