1. Nucleic acids introduced into the rumens of calves, or incubated with calf, sheep or cow rumen contents in vitro, were rapidly destroyed.2. The degradation products formed were separated and identified by means of column chromatography on Sephadex G-10 Dextran gel and thin-layer chromatography on cellulose.3. In vitro, RNA was rapidly (within 1 h) converted into ultrafilterable oligo-and mono-nucleotides, nucleosides, purine and pyrimidine bases. After 4 h, only the bases xanthine, hypoxanthine and uracil remained, having increased at the expense of the other constituents.4. DNA gave similar products but with a much greater proportion of ultrafilterable oligoand mono-nucleotide material which remained as a major component even after 4 h. The only bases present in appreciable amounts were thymine, hypoxanthine, uracil and xanthine.5. The same products accumulated temporarily in vivo, after addition of RNA or DNA to the rumens of calves, and were found also, in small amounts, in corresponding samples of duodenal digesta. The products disappeared from the rumen at a greater rate than could be accounted for by transfer to the duodenum.6. Cell-free preparations from calf rumen fluid contained enzymes which converted RNA and DNA into products which appeared to be ultrafilterable oligonucleotides.7. When ground hay was incubated with whole rumen contents the nucleic acids in the hay were degraded to a mixture of nucleotides, nucleosides and bases, almost as readily as were pure nucleic acids.