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The aim of this study was to investigate in vitro in a human hepatoblastoma cell line, Hep G2, the effect of ethanol (EtOH) toxicity. The ultrastructural changes were assessed by performing quantitative light and transmission electron microscopy. The second objective of this study was to define further EtOH-induced biochemical changes associated with mitochondrial function. In comparison with controls, after exposure to 80 mm EtOH cells showed: a threefold increase in length of mitochondria; proliferation, vesiculation and dilatation of smooth endoplasmic reticulum, and twofold increases in the size of lipid droplets and in their number/cell. Exposure of cells to two doses of EtOH augmented the ultrastructural alterations observed after a single dose. Cytoviability, assessed by metabolism of methylxanthine dye decreased significantly by (P < 0.0001) to 68% of the control after one dose and was further reduced after the second dose of EtOH (P < 0.001). Succinate dehydrogenase activity in cells treated for 24 hr with 80 mm EtOH was decreased to about 80% of control values after one 24-hr treatment with 80 mM EtOH and to about 55% of control values after two 24-hr exposures. This in vitro model of ethanol-induced cytotoxicity in Hep G2 cells is able to reproduce essential ultrastructural features of alcohol-related hepatitis, in humans, including steatosis and dose-dependent hepatocytotoxicity. The present work represents the first morphometric study to measure changes produced by EtOH exposure in human-derived liver cells.