In this work is shown that the repetitive high power microwaves is able to exert an inhibitory influence on the process of DNA and RNA syntheses in tumor cells of P-815 mastocytoma. This effect depends on pulse repetition rate. High power microwave pulses inhibit the process of transcription in tumor cells. No activation of DNA reparation system due to the irradiation of non-proliferating mononuclear blood cells was found. This indicates that the repetitive high power microwaves are not able to initiate single-filament rupture in DNA of tumor cells. The conformation of transcription enzymes is assumed to be changed under the influence of the microwave irradiation that makes for significant inhibition of RNA synthesis.