The kinetics of hybrid formation between the RNA of BrdU-activated endogenous guinea pig virus and the DNA of leukemic, normal, or BrdU-activated guinea pig cells were measured by the technique of RNA.DNA hybridization in DNA excess. The results suggest that virus-specific sequences representing some 60-70% of the viral genome are unique (2-3 copies per haploid cell genome), while the remainder (30-40%) are reiterated (147 copies), and that the reiterated virus-specific DNA may be composed of more than one species, each having a different reiteration frequency. No difference was found in the quantity of viral DNA sequences contained in normal, leukemic, or bromodeoxyuridine-activated guinea pig cells. These data are considerably different from those reported for exogenous (infectious) oncornaviruses, where cells infected or transformed by exogenous RNA tumor viruses have been shown to contain increased amounts of virus-specific DNA. The data reported here are consistent with the contention that preexisting viral genes are activated by bromodeoxyuridine treatment. Results of hybridization experiments done at different DNA/RNA ratios suggest that although the virus-specific DNA is partly unique and partly reiterated, the viral RNA does not contain any detectable internal reiteration. Total mass of the viral RNA sequences is around 0.7 to 1 x 10(7) daltons.