Single oral administration of octabromobiphenyls at 1,000 mg/kg, or two consecutive doses of 3,000 mg/kg, to rats produced liver enlargement. Light microscopic examination revealed hepatocellular hyperplasia, margination of basophillic cytoplasm, and foamy cytoplasmic alteration on the third day posttreatment. Laminated cytoplasmic inclusions developed seven days after treatment and subsequently disappeared about one week later. Under electron microscopy, the cytoplasmic margination corresponded to peripherally displaced granular reticulum and the foamy cytoplasm was recognizable as proliferating agranular reticulum with depletion of particualte glycogen. The cytoplasmic inclusions were identified as myelin configurations enclosing lipid bodies, membrane-bound vacuoles with whorled figures, and vesicular agranular reticulum. In the early developmental stages, the myelin figures were studded with ribosomes and associated with the granular reticulum.