The effect of the plasticizer on the extended linear calibration curve and on the selectivity of a calcium selective electrode with ETH 1001 ionophore was studied as a function of calcium activity in the internal solution. (2-Ethylhexyl)sebacate (DOS) and o-nitrophenyloctyl ether (o-NPOE) were used as plasticizers. The poly(vinylchloride) membrane also contained potassium tetrakis(4-chlorophenyl)borate. The linear part of the calibration curve of the electrode with o-NPOE is longer and the detection limit is lower compared to values for the electrode containing DOS as the plasticizer. The optimal activity of free Ca2+ and Na+ in the internal reference solution was 10(-4) and 10(-1) for the membrane with DOS and 10(-6) and 10(-1) for the membrane with o-NPOE, respectively. The repeatability of the response for electrodes with the lowest detection limit is similar in the case of both plasticizers. The selectivity coefficients were determined for electrodes having activities of calcium ion in the internal solution in the range from 10(-2) to 10(-10). The properties of the electrodes can be correlated with the transport properties of their membranes.