Just as after 70% hepatectomy, the activity of the 7.1 S DNA polymerase, but not of the 3.2 S polymerase, is elevated in liver nuclei from unoperated animals [rat] in which hepatic DNA replication has been induced with a mixture of biochemicals or by a dietary manipulation. As with regenerating liver, the stimulated intact livers show a relationship between the increases in the enzyme activity and thymidine incorporation in vivo over a wide range of hepatic responses. These observations are consistent with a role for the 7.1 S activity in nuclear DNA replication. Cytosine arabinoside 5''-triphosphate and novobiocin can be used to distinguish between the 3.2 S and 7.1 S polymerases from nuclei of stimulated intact liver and of regenerating liver.