[en] The method and results of the study of positive ions yield in the mass-spectrometric investigations by electron impact ionization of molecules of guanine, adenine and thymine are described. Mass spectrum obtained by electron impact has been measured in a crossed electron-molecule beam experiment ionization at the electron energy 70 eV. The ionization energy of the molecule and the appearance potential of some fragment ions were obtained. The mass spectra and fragmentation schemes of adenine and thymine molecules by electron impact were analyzed. The analysis of measured mass spectra is carried out, schemes of fragmentation of guanine, adenine and thymine molecules are proposed, which illustrate the most probable channels for the formation of ionic fragments in an electron impact, when the energy of the incident electrons is much higher than the ionization potential of the molecule. An explanation of the behaviours of the obtained temperature dependences of ions can be due to the difference in the character of the formation and fragmentation of the guanine, thymine and adenine molecules by electron impact. (authors)