Effects of aflatoxin B1 (AFB1) administration (7 mg/kg body weight, i.p.) on rat hepatic mitochondrial respiratory components have been examined. Succinoxidase and cytochrome oxidase activities were decreased in liver mitochondria isolated from rats 12-24 h after AFB1 treatment. Both enzyme activities returned to normal levels after 48 h. Glutamate dehydrogenase and beta-hydroxybutyrate dehydrogenase activities did not show any alterations up to 24 h and thereafter increased at 48-72 h. Succinate dehydrogenase activity was impaired by 41% at 12 h and thereafter was found to be normal. The intramitochondrial cytochrome b content declined at 24-72 h, whereas cytochrome aa3 content was decreased maximally at 72 h after AFB1 administration. These observations on mitochondrial enzyme activities and cytochrome contents correlate well with our earlier observations made on hepatic mitochondrial respiratory rates after AFB1 treatment. The impairment of respiratory functions possibly results from membrane damage and selective modification of gene expression in mitochondria imparted by AFB1.