Abstract: Anew PVCmatrix membrane double-barrelled calcium ion-selective microelec-trodc based on liquid ion exchanger has been designed and constructed. The tip diameter of about 2.5μm as well as lower selectivity coefficients for K⁺, Na⁺ and Mg²⁺ are adequate for intracellular measurements of Ca²⁺ activities.The inner wall of the selective channel was made by hydrophobic treatment with alcohols. By means of this microelectrode some physiological phenomena related to Ca²⁺ activities have been studied, and Ca²⁺ concentrations in clinical microsamples have also been determined.