Metolachlor, a commonly used herbicide in the Midwestern USA, functions by inhibiting chlorophyll and protein synthesis in target plants. Herbicide exposure has led to detrimental effects in several organisms, affecting their growth and behavior; however, its mechanism of action in nontarget organisms is not yet clear. The EPA does not currently have enforceable regulations for maximal limits allowed in drinking water. Previous growth studies from our lab have demonstrated that increasing metolachlor concentrations and increasing time of exposure results in decreased growth of liver cells. The objective of this study was to elucidate a mechanism for this decrease of HepG2 cell growth after herbicide exposure. Results show that metolachlor at environmentally relevant levels (50-100 ppb) that previously led to decreased cell number does not lead to cell death by either necrosis or apoptosis. However, it was demonstrated that the levels of the retinoblastoma protein including two of its hyperphosphorylated forms are decreased in metolachlor exposed cells possibly leading to cell cycle arrest. The levels of another protein involved in cell cycle progression, p53, a mediator in the DNA damage response of cells, was not significantly altered except at the highest level of metolachlor (1,000 ppb) and after a 72-h exposure. These results suggest that the decrease in cell number after low-level metolachlor exposure is most likely due to an alteration in the cell cycle and not due to cell death in human liver cells.