An electrochemical method for studying channels in porous rocks and similar insulators, has been developed. In principle, the method consists of displacing a nonconducting liquid from the pores by an inflow of an ionically conducting solution. The conductance is monitored continually as the rate of flow through the rock is gradually increased. The rate of conductance increase is interpreted in terms of a simple model, based on Poiseuille flow through a capillary network, to provide information about the sizes and numbers of large pores in the rock. Three rock samples have been analysed.