The incorporations of [3H] leucine into protein and [3H] uridine into RNA in mouse liver were increased when PASPA was given to mice at a dose of 30 mg/kg for 4 successive days. The RNA polymerase activity, especially the RNA polymerase II activity in the solubilized liver nuclear fraction of PASPA-treated mice was also increased. In vitro experiment demonstrated that PASPA increased the RNA polymerase activity significantly in mouse liver nuclei at a concentration of 1 microgram/ml. These results suggest that the enhancement of RNA polymerase activities, particularly RNA polymerase II activity, induced by PASPA treatment is responsible for the increase in synthesis of protein and RNA in mouse liver tissue.