Changes in hepatic ultrastructure were observed in rats given hexachlorobenzene (HCB) as a component of the diet in concentrations of 5, 10, and 25 ppm for 3, 6, and 12 months. A significant change was in the cellular regions normally containing smooth endoplasmic reticulum (SER). Sometimes the SER proliferated and at other times it appeared to be replaced by quantities of storage product presumed to be glycogen. Proliferation of SER was often associated with the smaller doses of HCB and the increase in storage products with the larger doses of HCB. However, dose relationships were variable. The most significant changes were in mitochondria which became elongated or swollen. Swollen mitochondria were occasionally as large as nuclei. Usually only a fraction of the mitochondria in a cell appeared abnormal. Storage bodies 1 to 4 mum in diameter developed in some cells. These bodies were surrounded by a double membrane derived from the endoplasmic reticulum and may have been the partly digested remains of degenerated mitochondria.