Radioactively labeled mouse mammary tumor virus (MuMTV) 60-70S RNA, obtained from virions grown In both murine and feline cells, was employed In molecular hybridization experiments to detect MuMTV-related sequences In the DNA’s of rats (Rattus norveglcus). With the use of relaxed conditions of hybridization and assay for RNA-DNA duplexes, all strains of laboratory rats and feral rats examined were shown to possess endogenous MuMTV-related DNA sequences In the low repetitive range. These sequences were related to approximately 20% of the MuMTV genome and exhibited a melting temperature (Tm) approximately 5° C lower than MuMTV-specific proviral sequences In murine (Mus musculus) DNA’s. Certain colonies of the F344 strain of rat (Fischer) contained animals whose DNA’s possessed additional MuMTV-related sequences. These sequences were related to the non-germ-line-transmitted, tumorassociated (TA) sequences of the highly oncogenic MuMTV (C3H). They were found In the DNA of some F344 rats and a cloned established F344 rat embryo cell line at a frequency of approximately one copy per haploid genome and exhibited a Tm 9° C lower than that of hybrid duplexes formed between radioactive MuMTV TA-sequence RNA and C3H mouse mammary tumor DNA. The DNA’s of rats, therefore, contained two sets of sequences that were related to sequences of the MuMTV genome: One set was germ-line transmitted, whereas the other set appeared to be transmitted in some rats via a non-germ line or infectious process.