M-DNA is a complex of metal ions such as Zn²⁺ with duplex DNA. Previous results showed that the fluorescence of a donor fluorophore was quenched when an acceptor fluorophore was placed at the opposite end of a short M-DNA duplex. In order to investigate further the molecular wire behaviour of M-DNA, 30-mer duplexes were constructed with fluorescein as donor and rhodamine, pyrene and the cyanine dyes, Cy5 and Cy5.5 as acceptors. Good quenching was observed in all cases even though the efficiency of resonance energy transfer was calculated to be < 5%. The distance dependence of quenching was investigated by preparing doubly-labelled duplexes ranging in length from 20 to 1,000 base pairs. Upon formation of M-DNA significant quenching of the fluorescence of the donor fluorophore was observed in duplexes up to 500 base pairs in length. The amount of quenching decreased with increasing length of the duplexes with a shallow distance dependence. The results are consistent with an electron transfer mechanism in which the electron hops between metal centers. This process can occur efficiently over long distances.