1. [1−14C]Ethyl carbamate, ethyl [carboxy14C]carbamate, [1−14C]ethanol and sodium hydrogen [14C]carbonate were injected intraperitoneally into C57 mice, and nucleic acids and proteins were separated from the liver and lungs with phenol as described by Kirby (1956). 2. Chromatographic analysis of the hydrolytic products of the urethane-labelled RNA showed the presence of a single radioactive compound differing in behaviour from the major pyrimidine nucleotides and purines. 3. The products from RNA labelled by [1−14C]ethyl carbamate or ethyl [carboxy14C]carbamate appeared chromatographically identical but could not be detected in the RNA of mice given [1−14C]ethanol or sodium hydrogen [14C]-carbonate. 4. The labelled product appeared to be the ethyl ester of cytosine-5-carboxylic acid formed by the reaction of urethane with RNA in vivo. 5. A direct reaction between labelled urethane or the labelled metabolite of urethane, [1−3H]-ethyl N-hydroxycarbamate, and RNA was not detected.

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