Abstract: Anew PVCmatrix membrane double-barreled calcium ion-selective microelectrode 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 in-tracellular measurements of Ca⁺ activities.The inner wall of the selective channel was made to be hydrophobic by treatment withalkyl-alcohols.By means of this microelectrode some physiologicalphenomena related to Ca²⁺ activities have been studied,and Caconcentrations in clinical microsamples have also been determined.