The activity of an endonuclease(s) acting on double-stranded, ultraviolet-irradiated, and 2-acetylaminofluorene-bound DNA but not on double-stranded undamaged DNA triples within two hr after partial hepatectomy. Although the activity drops between four and six hr after operation, it remains above levels measured in livers of nonhepatectomized rats until 36 hr after operation. Between 36 and 48 hr after operation, the enzyme activity drops below the levels in liver of nonhepatectomized rats and then rises slowly to reach levels observed in nonhepatectomized animals between 48 hr and seven days after the operation. Studies on the effect of actinomycin on the activity of crude enzyme and on the incorporation of [14C]leucine and [14C]valine on the purified enzyme indicate that the increase in enzyme activity results from de novo synthesis. Eight % of endonucleolytic activity detectable in the crude homogenate is inhibited by an hyperimmune serum prepared against the purified enzyme. By adjusting the time of injection of 2-[14C]acetylaminofluorene with respect to the levels of enzyme activity after partial hepatectomy, an inverse correlation between binding and enzyme activity was demonstrated.