Detection of polychlorinated biphenyls (PCBs) in the environment in the late 1960s along with increasing scientific data on their detrimental health effects led to a voluntary ban on these chemicals for industrial uses except in closed systems such as dielectric fluids. Commercial mixtures of the chemicals were manufactured extensively from the mid-1950s to the 1970s for use as flame retardants, lubricants, and dielectric fluids. It was not until the late 1970s that the manufacture of PCBs ceased. Environmental contamination of PCBs, however, continues to be a pervasive problem because of their lipophilic nature and resultant bioaccumulation in the food chain. PCB 118 (2,3',4,4',5-pentachlorobiphenyi) is one member of the chemically inert group of compounds composed of 209 congeners, effects of which are described in current work as a quantitative analysis of hepatocyte ultrastructure using stereology techniques. We determined the mean volume fractions of mitochondria and glycogen particles in the liver cells following exposure of PCB 118 to weanling Sprague-Dawley rats.