This invention relates to a new method of synthesis of biologically active substances of determined structure directly in the cells of living organisms containing specific RNA or DNA molecules of determined sequence. The method is based on the hybridization of two or more oligomers bound with biologically inactive precursors of biologically active substances to specific RNA or DNA in vivo in the cells of living organisms. After hybridization of the oligomers to RNA or DNA the biologically inactive precursors bound to the 5' and/or 3' ends of the oligomers can interact with each other to make biologically active form of the substances. This changing of properties is due to chemical reactions which bind the biologically inactive precursors through a chemical bond into a biologically active form of the whole compound.