A series of V- and Y-shaped nucleic acids, related to the splicing intermediates derived from S. cerevisiae actin pre-mRNA, were prepared. The effects of such branched nucleic acids (bNAs) on the efficiency of in vitro pre-mRNA splicing in yeast were studied. The exogenous bNAs each effect the efficiency of splicing, yet to different degrees, depending on the sugar composition and topology of the molecules. Y-shaped RNAs inhibited the formation of mRNA (i.e. RNA splicing) to the greatest extent.