Plasmid pATV8 carrying Rous sarcoma virus (RSV) DNA with one long terminal repeat (LTR) or plasmid pAPrC carrying complete RSV proviral DNA was injected into fertilized eggs of Xenopus laevis. The fate of the exogenous DNAs was followed during embryogenesis. In both cases a new form of DNA, comigrating with endogenous DNA, began to appear shortly after the injection. This new form represented a linear dimer of the injected plasmid that had formed by ligation of linear monomers of the plasmid DNAs. All forms of the exogenous pATV8 DNA disappeared gradually during neurulation. No traces of virus-specific RNA were ever found in any of the following stages. The intensity of replication of plasmid pAPrC DNA in fertilized eggs was greater than that of pATV8 DNA and, in contrast to pATV8, a small amount of pAPrC persisted through the neurula till higher stages. In some experiments virus-specific DNA was even observed in 6-day-old embryos. Digestion with Nsi I and Sac I proved integration of the plasmid into the frog genome. At the stage of gastrula and at later stages a weak signal of viral RNA was also detected.