Purified rat liver nuclei were incubated in the presence of a labeled deoxyribonucleoside triphosphate and bleomycin (an antitumor agent) or N-methyl-N-nitrosourea (MNU, a direct-acting carcinogen) to compare their abilities to induce DNA repair synthesis. It was found that bleomycin induced the incorporation of [3H]dTMP and, to a lesser extent, [3H]dCMP and [3H]dAMP, whereas MNU induced incorporation of [3H]dAMP exclusively. The bleomycin-induced DNA repair was linear with time, whereas the MNU-induced repair was more complex, requiring an induction period and lasting only 90 minutes. The extent of incorporation measured after a 15- or 30-minute preexposure to bleomycin was proportional to the time of preexposure and the repair reaction was completed within 30 minutes. The extent of incorporation measured after preexposure to MNU increased less than proportionally with the time of preexposure and the repair reaction lasted a total of approximately 90 minutes. The interactions between bleomycin and MNU damage and repair were also examined. It was found that, following preexposure to MNU and subsequent repair in the presence of bleomycin, the repair activities induced by the two compounds were not additive. Our data are in agreement with previous studies on the base excision induced by bleomycin and MNU and suggest that the in vitro nuclear system reflects the physiological changes induced by the interaction of a compound with DNA.