Calmodulin, a low molecular weight Ca2 + binding protein, regulates a large number of cell activities including cell division. Previous studies from our laboratory indicated excessive accumulation of Ca2 + in hepatocytes succeeded by rapid glycogen breakdown and suppressed cell division in rats receiving CCl4 after previous dietary exposure to 10 ppm chlordecone. Since calmodulin plays a major role in Ca2+ -regulated events and has been reported to be localized in mitotic apparatus during cell division, we have assessed subcellular distribution of calmodulin and estimated cytosolic phosphorylase a to indicate cytosolic free Ca2+ levels in livers of rats fed 0 ppm or 10 ppm (chlordecone) in the diet for 15 days before CCl4 (100 μ1/kg) administration to understand the role of Ca2+ -calmodulin in chlordecone + CCl4 toxicity. Hepatotoxicity was assessed by determining serum AST and ALT succeeded by histopathological observations of liver sections. Serum aminotransferases were significantly elevated 6 hr after CCl4 administration to normal rats and returned to control level by 24 hr. However, serum AST and ALT elevations were severalfold higher, and progressive increase was observed starting 4 hr after CCl4 administration to chlordecone rats. Histopathological observations of liver sections for necrotic, swollen and lipid-laden cells provided findings commensurate with the serum enzyme data. These data indicate that normal rats do recover from CCl4 hepatotoxicity. However, the CCl4 hepatotoxicity is progressive in chlordecone rats without recovery. In normal rats, CCl4 administration resulted in a slight increase in phosphorylase a starting at 6 hr. The elevation of phosphorylase a was many-fold higher, evident as early as 2 hr after CCl4 administration and was progressive with time in livers of chlordeconepretreated rats. Hepatic cyclic AMP levels were not increased in any treatment groups but instead were slightly decreased in the chlordecone + CCl4 combination treatment. This indicated that increased phosphorylase a is caused by increased cytosolic free Ca2+ but not cyclic AMP and occurred before necrosis of hepatocytes. Calmodulin levels were significantly altered after CCl4 administration to both normal and chlordecone-treated rats. Calmodulin levels in nuclear fraction decreased initially succeeded by a marked elevation after 12 hr of CCl4 administration. Although the cytosolic calmodulin levels were increased at all time points after CCl4 administration, the mitochondrial and, to some extent, the microsomal calmodulin contents were decreased. These changes in redistribution of calmodulin in subcellular compartments might be associated with altered Ca2+ levels. Based on this study and previous findings, altered Ca2+ homeostasis evidently is an early event that may lead to a number of biochemical perturbations in the liver cells, ultimately leading to the progressive phase of chlordecone-potentiated CCl4 hepatotoxicity. (HEPATOLOGY 1991;13:230–238).