The rate of virus and cell DNA synthesis was studied in human embryonic lung cells pre-treated with 5-iodo-2'-deoxyuridine (IdUrd) and exposed to cytomegalovirus (CMV) or medium. Analysis of DNA in CMV-infected cells following sequential 4 h pulses with 3H-thymidine indicated that a temporal relationship existed in the pattern of virus and cell DNA synthesis. The pattern of DNA replication in infected cells resembled that of a typical cell cycle, whereas the rate of cell DNA synthesis in uninfected cells remained low throughout the study. Increased rates of cell and virus DNA synthesis began concomitantly at 16 h post-infection and reached a maximum at 36 h post-infection. The rate of DNA synthesis then declined and remained at lower levels until 48 h post-infection. This was subsequently followed by a second increase in the rate of cell and virus DNA synthesis. The rates of cell and virus DNA replication were similar throughout the study in that increased and decreased rates of synthesis occurred simultaneously. It was of interest to note that CMV induced cell DNA replication in IDUrd arrested cells; in contrast, addition of fresh serum did not induce a similar increase in the rate of DNA synthesis in IdUrd arrested, but uninfected, cells.