The lateral and suprasylvian gyris of cats have been stimulated anodically by surface electrodes for a total of 40 h by biphasic square waves. Peak currents were ±6.0 mA or 1.0 mA,·to peak. Control electrodes were never stimulated. Electrode materials were low-temperature isotropic carbon and metals (Rh, Pt, Au). Reductions in tissue impedance quickly followed the onset of stimulation. The impedance change was related to stimulus current and to the incidence of motor seizures but not clearly to the electrode material. Utilizing concentric electrodes, the impedance change was not localized to the stimulating electrode – tissue interface. No protection from stimulation-associated impedance modifications was provided by the systemic administration of a material of high osmolarity (Mannitol) but the usual impedance decrease was not seen after systemic administration of a glucocortic