Effect of a single long-term (6 hrs) emotional-painful stress on DNA synthesis was studied in nuclei and mitochondria of heart and liver cells. Various systems of DNA synthesis in heart and liver cells were shown to respond dissimilarly to the stress. The rates of replication and reparative synthesis of nuclear DNA were increased while synthesis of mitochondrial DNA was unaltered in heart cells within the first day after the stress. In liver cells the reparative synthesis of nuclear DNA was also increased, whereas the rate of its replication in nuclei and mitochondria was distinctly inhibited for a long time. Thus, the systems of DNA synthesis in liver cells proved to be more sensitive to extreme stress-reactions as compared with heart cells. Effect of the stress on the systems of DNA synthesis in specific and connective tissue cells of liver and heart is discussed. At the same time, mitochondria are localized in specialized cells of both heart and liver tissues. The data obtained suggest that inhibition of the mitochondrial DNA synthesis is realized in hepatocytes.