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The use of heart pacemakers and other implantable electronic devices is increasing. The hybrid cell, as a power source, consisting of a sacrificial anode and a catalytic cathode for tissue oxygen reduction has been successfully employed to pace heart‐blocked dogs. The life and biocompatibility of the cell are determined by the anode material loss in the recipient's body. Corrosion rates of Al anodes implanted in dogs have been studied using the d‐c polarization technique of Greene and Stern. The difference in the measured apparent corrosion rates is explained. The effect of extensive pitting of Al under galvanostatic loading conditions and its effect on the linear and Tafel curves are discussed. Preliminary data from in vivo a‐c impedance measurements are presented in an effort to correlate them to the corrosion parameters. Problems associated with the a‐c bridge measurements include the elimination of the tissue impedance (exhibiting possible dielectric relaxation) between the measuring electrodes. It is shown that it is possible by careful selection of the electrode geometry to minimize the effects of tissue and auxiliary electrode impedances on the metal‐tissue interface parameters of the test electrode.

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