Changes in the resistance and tip potential of open-tip glass microelectrodes as function of the polarity and intensity of an injected current are presented and analyzed. These experimental results are explained on the basis of a quantitative diffusion model which includes an electroosmotic effect through the microtip. This study also results in a practical method to improve microelectrode selection. The method, based on the change of resistance as a function of the polarity of the injected current, allows immediate rejection of incompletely filed microelectrodes.