Male Leeds rats were given single oral doses of 150 or 300 mg/kg body weight of 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB). They were sacrificed 24 or 48 h after treatment and their hepatic tissues examined by electron microscopy. The development was observed of an unusual cytoplasmic change, which consisted of large perinuclear areas of decreased hyaloplasmic density, devoid of glycogen or organized structures, which displaced the organelles to the cell periphery. This arose by the formation of large glycogen lakes, which coalesced and then lost their glycogen content, and this was accompanied by nuclear irregularity and shrinkage. Other changes, affecting the endoplasmic reticulum and the cell surface, appeared to be similar to those induced by chronic azo dye feeding. It was concluded that the acute lesion observed may limit any specific responses of the hepatocytes to the presence of 3'-MeDAB.