Five kinds of monensin derivatives were synthesized by chemical modifications of the natural carboxylic polyether antibiotic monensin. Their usefulness as active compounds (Na⁺-ionophores) for Na⁺-selective poly(vinyl chloride)(PVC)-matrix membrane electrodes was examined. The electrodes based on monensin ester derivatives exhibited high Na⁺-selectivity. Among them, the electrode based on the highly lipophilic monensin dodecyl ester(log K⁰ (partition constant)=8.7) had enough Na⁺-selectivity to determine the Na⁺-concentration in serum(log K^pot_NaK=-1.1, log K^pot_NaMg=-4.0 and log K^pot_NaCa=-3.6); the electrode gave a Nernstian response (59.0mV/pNa) ranging from 5×10^-6 to 1M Na⁺. The response potential of this electrode to a 0.1M Na⁺ solution drifted within ±0.01mV/h. The values of the Na⁺-concentration in artificial and real serum samples were successfully determined with the monensin dodecyl ester-based electrode using diisodecylphthalate as the electrode membrane solvent.