[en] Nuclear DNA can be empirically subdivided into three populations: (1) bulk DNA or low salt-soluble DNA (75%), (2) high salt-soluble DNA (23%), and (3) matrix DNA which remains tightly bound to the nuclear matrix (2%). Newly replicating DNA is associated with the nuclear matrix in regenerating rat liver. To study the incorporation of DNA precursors into replicating DNA via the salvage vs the de novo pathway, 100μCi [³H]thymidine (³H-Thd) and 5mCi [³²P]orthophosphate (³²P/sub i/) were injected into the hepatic portal vein of partially hepatectomized rats. Increasing time of ³H-Thd incorporation showed the label is chased from matrix DNA to bulk DNA. After a 10 min pulse, 13% of the total specific activity is associated with bulk DNA and 57% with matrix DNA. After 30 min, 32% and 36% of the total specific activity remain associated with bulk and matrix DNA, respectively, indicating that most of the ³H has been chased from the matrix DNA. In contrast, after injection of ³²P/sub i/, the amount of label in matrix DNA increases to a maximum at 30 min and only then begins to decrease. At 10 min the specific activity/total specific activity of bulk DNA is 7% and of matrix DNA is 66% vs 8% and 82% after 30 min. The kinetic pattern of ³²P/sub i/ incorporation differs dramatically from that of ³H-Thd suggesting (a) the incorporation of de novo precursors lags significantly behind that of precursors entering through the salvage pathway, or (b) there may be two distinct classes of replication forks