We study the influence of grain size distributions of 2D and 3D polycrystals on the electrical response of grain boundaries by means of numerical calculations based on Kirchhoff networks. The GB contribution to the overall conductivity is compared to that of the bricklayer model in order to estimate the validity of such model. We thus quantify the error committed when using the BLM to extract the intrinsic GB resistivity of 2D and 3D real polycrystals from impedance measurements. (c) 2006 Elsevier B.V. All rights reserved.