N-methyl- and N-nitrosourea (NMU) methylate and carbamoylate DNA, RNA and chromatin proteins. The paper is concerned with an analysis of the MNU-induced changes in the synthesis of RNA in isolated nuclei, chromatin, and in protein-free DNA. It was found that NMU decreases the rate of the RNA synthesis in all the templates used. It was also found that a 5-10 times greater concentration is required to attain the same effect in the nuclei as in chromatin. Comparison of the effects of NMU, KNCO and KCl on the template activity of chromatin allows the conclusion that the processes of methylation rather than of carbamoylation of the template are responsible for the decreased RNA synthesis. Sedimentation of the RNA synthesized over the saccharose density gradient demonstrated that the changes in the synthesis are a consequence of the decreased molecular mass of the RNA-transcript, while the number of the RNA molecules synthesized is approximately the same in control and experiment.