We have studied the effects of relative humidity on the conductance of the assembly of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA molecules. The exponential dependence of conductance on the relative humidity is observed and explained in terms of ionic conduction through the hydration layers around DNA. The poly(dG)-poly(dC) specimen has larger sensitivity to the relative humidity and smaller activation energy for the ionic conduction than the poly(dA)-poly(dT) specimen. Our results show that the conductance of a specimen consisting of multiple DNA molecules might be strongly affected by the relative humidity. (C) 2002 Elsevier Science B.V. All rights reserved.