Isolated nuclei from HSV-1-infected rat embryo fibroblasts (RE-2) were capable of incorporating labeled nucleotides into viral DNA in the presence of four deoxy-ribonucleoside triphosphates, Mg<sup>++</sup> and ATP. The reaction stopped after 10–15 min, and the product degraded upon prolonged incubation. The viral DNA synthesized by the isolated nuclei had heterogeneous sedimentation profiles (S<sub>20,w</sub> = 4–40S) in the neutral sucrose density gradient; upon denaturation by NaOH, the viral DNA sedimented in the 4–7S region of the alkaline sucrose density gradient. It had a CsCl buoyant density of 1.725 g/cm<sup>3</sup> and was complementary to homologous DNA from purified virions but not to heterologous cell DNAs. Under identical conditions of incubation, isolated nuclei from uninfected cells incorporated [<sup>3</sup>H]-TMP to cell DNA (ρcscι= 1.700 g/cm<sup>3</sup>) but to a much lesser extent than the HSV-1-infected nuclei. The DNA synthesized by uninfected nuclei hybridized with cell DNA but not with HSV-DNA.