Viral and cellular DNA synthesis were examined in synchronized HeLa cells infected at various stages in the cell cycle with adenovirus type 2. Viral DNA synthesis began approximately 9 hours after infection, whether cells were infected during S or G1. When viral DNA was synthesized during G1, premature replication of cellular DNA did not occur, and its subsequent initiation at the beginning of S was prevented. When viral DNA synthesis began after the onset of S, cellular DNA replication was partially, but not completely, inhibited. Nucleohistones were synthesized only in association with cellular DNA. © 1969.