All possible dimers of the title modified bases with native nucleobases [10 dimers from 3-methylated 6-aminouracils (3sau) and 20 from 5-methyled 6-aminouracils (5sau), respectively have been calculated by ab initio method (Hartree-Fock method, 3 21G basis set). We have found two potential duplexes of 5sau and three possible duplexes of 3sau. Altogether seven dimers containing one or two bifurcating H-bonds have been found. Later on, five triplexes from ten possible calculated dimers have been found. In two of them the amino group of 6-aminouracil moiety takes part in H-bonding and there are H-bonds, too, between the first and third base of the triplexes causing an extra stabilization.