3-Aminobenzamide and benzamide, two potent inhibitors of poly-(ADP-ribose)-polymerase increase the frequencies of SCEs in Chinese hamster ovary cells in a dose-dependent manner. SCEs were studied in cells in which the inhibitors were present either during the first cell cycle or the second cell cycle or both. Most of the induced SCEs were found to be formed during the second cell cycle in which BU-containing DNA was used as template for DNA synthesis. In cells which were pregrown for 4 cell cycles in the presence of BrdUrd, in order to obtain both sister chromatids bifiliarly substituted with BU in their DNA, it was found that the presence of inhibitor even in the first cell cycle increased the frequencies of SCEs. It is concluded that the incorporated BrdUrd plays an important role in the origin of spontaneous and induced SCEs. 3-Aminobenzamide alone or benzamide in the presence of BrdUrd during culture, did not increase the frequencies of mutations to HGPRT- in these cells.