Rats were given a single dose of aflatoxin B1 lethal to 50% of the animals (7.20 mg/kg). Their livers were examined histochemically in correlation with sequential histological lesions. Early periportal liver cell necrosis and marked biliary cell proliferation were observed. Periportal cytoplasmic glycogen and RNA depletion occurred during this early period and subsequently extended to the whole lobule. The enzymes investigated decreased or disappeared in the periportal area; but alkaline phosphatase increased strikingly in the centrolobular area, whereas canalicular adenosinetriphosphatase completely disappeared throughout the liver lobule. The histochemical changes reverted to normal after cessation of the necrosis. Histochemical techniques were more sensitive in detecting the vulnerability of the periportal parenchyma to aflatoxin. After the necrosis, regenerative foci appeared. They showed a variable content in glycogen and RNA and were characteristically enzyme deficient. This reflects the immaturity of regenerating hepatocytes. These early foci subsequently disappeared and are thus considered irrelevant to hepatomagenesis.