Previous work has shown that daunomycin reacts with DNA of procaryotes and eucaryotes. Results with intact yeast cells indicate that mitochondria are preferentially inhibited by the drug at a concentration of about 10 μg/ml in most strains. Since the drug does not induce the petite mutation, at least over a period of one growth cycle, the replication of mitochondrial DNA is apparently unaffected. In a fermentable medium in the presence of the drug, cells develop only about 40 % of the respiratory activity of untreated cells. Incorporation of [ 14C]uracil into cells is depressed by the drug at low concentrations which inhibit growth of cells in a non-fermentable medium. In similar circumstances, incorporation of radioactive amino acid into proteins is unaffected in the short term. It was concluded that mitochondrial RNA synthesis was primarily affected by daunomycin. A preparation of rat liver mitochondria which incorporated [ 3H]UTP for at least 60 min, was inhibited about 30 % by addition of 10 μg/ml of the drug. Higher concentrations of the drug up to 20 μg/ml did not further depress the RNA synthesis. It was suggested that daunomycin inhibits the synthesis of a particular fraction of mitochondrial RNA.