The simultaneous detection of electrochemical and thermal responses of polypyrrole in 1 M NaClO4 aqueous solution (pH = 5) was carried out. Thermal measurement with cyclic voltammetry and galvanostatic electrolysis revealed exothermic and endothermic changes due to anodic and cathodic currents, respectively, on the surface of the polypyrrole electrode. The proportional relation between the heat due to electrolysis and electric quantity or current was confirmed in both the exothermic and endothermic processes. This electrochemical Peltier heat could be attributed to the entropy change in the electrochemical doping and dedoping of ClO4- ions. It suggested the occurrence of a decrease and an increase in entropy in the doping and dedoping processes, respectively. The value of the entropy change in the dedoping of ClO4- ions could be evaluated on the basis of the entropy change in the anodic oxidation of Fe(CN)(6)(4-) ions.