Aflatoxin, a potent hepatotoxin, causes rapid inhibition of RNA polymerase activitiy in isolated rat liver nuclei. Its hepatotoxicity is accompanied by an injury of periportal parenchymal cells. Rats fed a diet marginally deficient in choline are protected against the acutely toxic action of aflatoxin B1. This diet does not prevent the rapid inhibition of RNA polymerase or the changes in nucleolar morphology following the administration of aflatoxin B1. The diet does however largely prevent the proliferation of smooth endoplasmic reticulum and the necrosis of periportal parenchymal cells. The diet appears to offer the possibility of separating those changes in the liver associated with hepatotoxicity from those associated with hepatocarcinogenicity.