The influence of metal cations (M = Cu+, Ca2+ and Cu2+) coordinated to the N7 of guanine on hydrogen bonding and aromaticity of the guanine-cytosine base pair has been analysed with the help of delocalization indices using the B3LYP functional. Our analysis shows that the strengthening of the N1⋯N3 and N 2⋯O2 hydrogen bonds and the weakening of the O 6⋯N4 hydrogen bond is mainly caused by the modification of donor-acceptor (covalent) interactions rather than to a significant change of electrostatic interactions. On the other hand, the increase of the aromaticity of the guanine and cytosine six-membered rings because of the interaction with Cu+ and Ca2+ is attributed to the strengthening of hydrogen bonding in the guanine-cytosine pair. The observed reduction of aromaticity in the five- and six-membered rings of guanine due to ionization or interaction with Cu2+ is caused by the oxidation process that removes a π electron disrupting the π electron distribution. © 2005 Taylor & Francis Group Ltd.