The preparation in situ of a silver-silver chloride reference electrode includes anodic-cathodic pretreatment of a silver electrode in a phosphate buffer solution comprising 0.1 M sodium chloride. At pH 7.0 and 25°C and with optimally pretreated electrodes, the potential of the Ag/AgCl electrode was 45.0 mV (vs. SCE) with a reproducibility of 0.2%. In a period of 30 seconds following a delay of 2 to 10 seconds, the electrode shows a very stable potential that changes only 0.2 to 0.3%. The electrode potential changes 0.3 mV/OC. The electrode shows nernstian dependence of the potential to the sodium chloride concentration. At the physiological level! the only electrochemically active compound that significantly influences the potential of the electrode is reduced glutathione. Paracetamol (0.2 mM) and uric acid (0.27 mM) alter slightly the electrode potential. The electrode parameters in plasma are close to those of phosphate buffer solution, whereas in artificial blood, the reproducibility and the stability of the potential decrease. The reproducibility and stability of the electrode potential in capillary blood was 1.4 and 3.7%, respectively.