An excess electron was attached to naphthalimide (NI) modified 12-mer oligodeoxynucleotides (ODNs) and the electron transfer in DNA was investigated by monitoring the transient absorption of NI radical anion (NI.-) during the pulse radiolysis. Formation of the transient absorption of NI.- was observed according to the reaction of e(aq)(-) with NI-modified ODN. Only 25% of e(aq)(-) reacting with NI-modified ODN were observed by the transient absorption of NI.-, which corresponds to electron transfer over no more than three base pairs, suggesting a low mobility of an electron attached to DNA. Electron transfer in DNA was also studied by gamma-radiolysis of ODN containing 5-iododeoxyuridine (U-I) as a second electron acceptor. Electron transfer in DNA was estimated by the protection of dehalogenation of U-I offered by NI during the gamma-radiolysis of NI- and U-I-modified ODNs where the spacing between the NI and U-I was varied. The protection effect became very low by the insertion of three or four A-T base pairs between NI and U-I. The results driven from both pulse radiolysis and gamma-radiolysis experiments were consistent with the low mobility of an excess electron in DNA, which is in strong contrast to the occurrence of the long-range hole transfer in DNA.