We report novel experimental results of chemical doping effect on Ag-doped DNA conductor. The prepared Ag-doped DNA samples were confirmed by EXAFS that Ag+ was doped into pi-way. In I-V measurements at room temperature, nonlinear I-V curves with hysteresis emerged and systematic change in electrical conductivity a was observed from 10(-10) to 10(-6) Omega(-1) cm(-1) under doping condition [Ag+]/[Base pair] = 10(-6)-10. The relation between the conductivity and dopant concentration showed a weak dependence a proportional to [Ag+](0.5). This 'weakly doping effect' may be caused by the long-range correlated randomness of sequence in DNA, in contrast with usual doped organic conductors. (C) 2004 Elsevier B.V. All rights reserved.