Liver nuclei isolated from male mice treated with the carcinogen N, N-diethylnitrosamine were examined for the homopolymer poly(adenosine diphosphate ribose) and for the activity of the conjugate polymerase. 2. At all levels of the carcinogen tested, a concomitant increase in both poly(adenosine diphosphate ribose) content and activity of the enzyme were found. 3. Both responses were transitory and dose dependent.