Hepatocytes from adult male Sprague-Dawley rats were isolated by the two-stage collagenase perfusion technique; 1 × 106 cells/plate were incubated in primary cell culture in Leibovitz's L-15 medium for 24 hr with or without various concentrations (12.5 to 400 μmol/L) of cardioactive cationic amphiphilic compounds such as propranolol, verapamil, sotalol, atenolol and procainamide. Propranolol and verapamil caused a significant release of lactate dehydrogenase (used as cytotoxic index in this study) in the culture media in a concentration-dependent manner, with LC50 values of 220 ± 10 and 224 ± 7 μmol/L, respectively. Atenolol, sotalol and procainamide had no effect on lactate dehydrogenase release. Electron microscopy of the hepatocytes showed that subtoxic concentrations of propranolol (12.5 to 125 μmol/L) and verapamil (12.5 to 100 μmol/L) induced multilamellar inclusion bodies after 24 hr of incubation. The two higher concentrations of propranolol (50 and 125 μmol/L) and 100 μmol/L of verapamil produced a significant decrease in the percentage of volume density of the mitochondria as quantitated by morphometrical analysis. An unusual feature of the electron microscopical changes with propranolol and verapamil was the presence of mitochondria within the multilamellar inclusion bodies. When these two drugs were used together or with subtoxic concentrations of amiodarone or desethylamiodarone, release of lactate dehydrogenase was significantly enhanced. No correlation was evident between the cytotoxic response and the volume density of cellular inclusions in hepatocytes treated with different concentrations of propranolol, verapamil, amiodarone or desethylamiodarone. Sotalol, atenolol and procainamide in concentrations up to 400 μmol/L did not produce any ultrastructural changes in hepatocytes after 24 hr of incubation. These results show that (a) cationic amphiphilic structure per se is not the only requirement for induction of multilamellar inclusions, (b) propranolol and verapamil can induce the formation of multilamellar inclusion bodies and cause a concentration-dependent release of lactate dehydrogenase from hepatocytes and (c) combination of different cationic amphiphiles in subtoxic concentrations can enhance cytotoxicity and increase the volume density of multilamellar inclusions. (HEPATOLOGY 1990;12:48–58).