Although replication of nuclear DNA is inhibited by cytarabine (ara-C), protein synthesis in the nucleocytoplasm appears to continue unabated for the duration of at least the time of the normal cell cycle. ara-C treatment of human leukemic cells resulted in increased mitochondrial membrane potential and adenosine-5'-triphosphate (ATP) production and increased activity of enzymes, coded on nuclear DNA (citrate synthetase), as well as of enzymes with subunits coded on mitochondrial DNA (cytochrome c oxidase). These mitochondrial changes occurred during a period of cell-cycle arrest, while cell size and cellular protein content continued to increase. These phenomena appeared to precede the ultimate cell death.