The anti-oxidant food additive, butylated hydroxytoluene (BHT), was fed to Sprague-Dawley rats at three concentrations: 0.2%, 0.4% and 0.8% for periods of 6, 12, 18 and 24 weeks, and the results were compared with corresponding groups treated with a potent carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA) groups, with olive oil, and with untreated control groups. BHT resulted in a significant increase in liver weight. The liver cells presented gradual vacuolization, cytoplasmic disintegration, "moth-eaten" appearance, ballooning degeneration, hepatocellular necrosis, aggregation of chromatin material around the periphery of the nuclear envelope, SER proliferation, RER clumping with broken cisternae, withered and autolyzed mitochondria, augmentation of lipid droplets and glycogen depletion. On the other hand, there was no sign of tumorigenicity. Whether or not BHT acts as a carcinogen in long-term administration may depend not only upon the organ system examined, but also on the strain of the animal used.