MP2/6-311G** and B3LYP/6-311G** studies of the intramolecular proton transfer in adenine, cytosine, guanine, and thymine has been performed, with the aim of evaluating the role of the reaction force as a global descriptor of the process. It turns out that the reaction force profile is quite an interesting tool to characterize reaction mechanisms. Indeed, in adenine and cytosine the proton transfer is assisted by an increasing electronic delocalization in the adjacent ring, whereas in guanine and thymine the attractive electrostatic interaction with the acceptor oxygen atom is strong enough to promote the transfer. (C) 2004 American Institute of Physics.