Na⁺ reduction induces contraction of opossum lower esophageal sphincter (LES) circular smooth muscle strips in vitro, however, the mechanisms by which this occurs is unknown. The purpose of the current study was to investigate the electrophysiological effects of low Na⁺ on opossum LES circular smooth muscle. In the presence of atropine, quanethidine, nifedipine and substance P, conventional intracellular electrodes recorded resting membrane potential (RMP) of -37.5 ± 0.9 mV (n = 4). Decreasing [Na⁺] from 144.1 mM to 26.1 mM by substitution of equimolar NaCl with choline Cl depolarized MP by 7.1 ± 1.1 mV. Whole-cell patch-clamp recordings revealed outward K⁺ currents, which began to activate at -60 mV using 400 ms stepped test pulses (-120 to +100 mV) with increments of 20 mV from holding potential of -80 mV. Reduction of [Na⁺] in the bath solution inhibited K⁺ currents in a concentration-dependent manner. Single channels with conductance of 49 - 60 pS were recorded using cell-attached patch-clamp configurations. Channel open probability was significantly decreased by substitution of bath Na⁺ with equimolar choline. A 10-fold increase of [K⁺ ] in the pipette shifted the reversal potential of the single channels to the positive by - 50 mV. These data suggest that Na⁺-activated K⁺ channels (K_Na) exist in the circular smooth muscle of opossum LES.