Incorporation of NAD into the nuclei was determined autoradiographically in cultured HeLa cells and in cryostat sections of rat organs by incubating them with 3H-NAD after fixation with various agents. Acetone fixation was the best to render the cells permeable to NAD while preserving the cell's enzymatic activity to incorporate NAD into nuclear macromolecules. Various evidence supported that such incorporation of NAD is due mostly to the synthesis of poly(ADP-ribose) on chromatin proteins. In the sections of rat jejunum and esophagus the rate of NAD incorporation was higher in the actively proliferating cell nuclei than in the differentiated cell nuclei within the same epithelia. These results suggested that the capacity of the cells to synthesize poly(ADP-ribose) is associated with cell growth and differentiation.