Impedance spectroscopy was used to solve the Pt electrode interface with metabolically active perfused living heart. Three impedance spectra were observed: the Warburg impedance (Z(Winfinity)), a single high angle constant-phase-element, and a thin-film impedance (Z(D)). When characterized again after cyclic change of ionic strength (and hence conductivity kappa) each interface had one of only two spectra, with exclusion of Z(Winfinity). The in vivo interfacial impedance spectrum is thus neither single-valued nor stable in time. Because metal\living tissue interfaces are obligatory circuit elements in biosensors and electrodes in heart and brain, the multiple-valued and thin-film character of its impedance are significant. (C) 2004 Published by Elsevier B.V.