The case of a reinforcing steel bar experiencing localized corrosion in a concrete cylinder has been modeled numerically to obtain the d-c macrocell current distribution and the associated electrochemical impedance response. The bar contains a small anodic and a large cathodic region. Cylindrical symmetry has been assumed. The d-c polarization problem was solved using finite-difference equations applied to a two-dimensional grid. The boundary conditions included polarization at the cathode, accounting for both activation and diffusional effects. The a-c impedance calculation was made for cylindrical counter electrodes. The computation used a two-dimensional equivalent circuit resistive grid representing the electrolyte and calculated for cylindrical geometry. The a-c polarization conditions at the bar surface were represented by discrete impedance elements, computed from the current densities obtained in the d-c calculation. Both activation and diffusional impedance components were considered as well as interfacial capacitance. The model was used to investigate macrocell current distribution and the applicability of electrochemical impedance to the measurement of corrosion currents in reinforced concrete structures with large cathode-to-anode area ratios. Predicted impedance spectra for concrete resistivity of 10 000 Ω-cm, with an effective oxygen diffusion coefficient of 10-5 cm2/s, and for various cathode/anode area ratios are presented. The results show that the distribution of the alternating current excitation emphasizes the impedance contribution of the cathodic reaction to the overall impedance spectrum.