The relationship between poly(ADP-ribose) synthesis and cytodifferentiation was studied in the well characterized Zinnia system, in which isolated mesophyll cells of Zinnia elegans transdifferentiate into tracheary elements (TE) in a suspension culture in the presence of both auxin and cytokinin. The rate of poiy(ADP-ribose) synthesis was measured in nuclei isolated from cells that had been induced to undergo transdifferentiation, and activation of such synthesis was observed before the appearance of TE during culture. In cultures without auxin or cytokinin, poly(ADP-ribose) synthesis appeared to proceed much more slowly. Treatment of cells with a potent inhibitor of poly(ADP-ribose) polymerase, namely, 6(5H)-phenanthridinone (PT), resulted in the blockage of TE formation and a decrease in the frequency of cell division. PT was very effective in interfering with transdifferentiation, in particular, when supplied between the 24th hour and the 36th hour of culture. Repair-type DNA synthesis, which has been proposed to participate in transdifferentiation, was suppressed by the treatment with PT. These results suggest that poIy(ADP-ribose) synthesis and subsequent repairtype DNA synthesis might play a critical role in the transdifferentiation of Zinnia cells.