Cardiac hypertrophy was produced in embryonic chicks by decreasing the incubation temperature from 38 degrees C to 32 degrees C on day 11. Increases in ventricular protein, RNA, and DNA support the cardiac enlargement. Cytochrome-c oxidase activity and citrate synthase activity were depressed in hypothermic ventricles by 63% and 56%, respectively. No significant differences were seen in enzyme activities in pectoralis muscles. The involvement of mitochondrial gene replication and transcription was evaluated using a cDNA clone for the mitochondrially encoded subunit III of cytochrome-c oxidase (CO III). Quantitative slot-blot analysis demonstrated that the relative CO III mRNA concentration was reduced in hypothermic ventricles. In contrast, the relative mitochondrial DNA concentration was increased in hypothermic ventricles. Taken together, these data indicate that a hypothermia-induced decrease in cytochrome-c oxidase activity is associated with a decrease in CO III mRNA, which is not coupled to a decrease in the mitochondrial DNA copy number. This dissociation of mitochondrial gene replication and transcription may provide a useful model for examining the regulation of mitochondrial biogenesis.