The fluorescent probe method was applied to determine the membrane potential of tobacco (Nicotiana tabacum L. cv Bright Yellow) cells that were either unadapted (NaCl-unadapted cells) or adapted to 50 mM NaCl (Na50-adapted cells). There were no deffrences in the bahavior of the membrane potential between the NaCl-unadapted cells and the Na50-adapted cells. The patch-cpamp technique was used to study and compare the characterristics of cation channels in the plasma membrance of NaCl-unadapted cells, Na50-adapted and Na100-adapted cells. The steady-state amplitude of the outward whole-cell currents decreased in the following order; NaCl-unadapted cells>Na50-adapted cells>Na100-adapted cells. There were no significant differences between the NaCl-unadapted cells and the Na50-adapted cells in terms of the K+/Na+ selectivity of these channels. These observations suggest that adaptation to salinity in reduced permeability of the K+ channels to both K+ and Na+. K+ channels in the plasma membrane were investigated using the patch-clamp technique. The outward rectifying whole-cell currents were reduced with increasing extracellular Ca2+ and were independent of intracellular Ca2+. The K+/Na+ selectivity were independent of Ca2+ concentration. These observations suggest that extracellular Ca2+ is an important factor in the regulation of ion permeability. The characteristics of Ca2+ binding on the plasma membrane were investigated by the micro-electrophoresis tecniqui. Comparison of the amount of Ca2+ bound in the presence of 30 mM NaCl with that in the presence of 30 mM KCl indicated that Na+ reduced the amount of Ca2+ bound to the plasma membrane of the NaCl-unadapted cells,but did not reduce the amount of Ca2+ bound to the plasma membrane of the Na50-adapted cells. These results suggest that adaptation to salt sterss results in increased resisrtance to the displecement of membrane-associated Ca2+ by Na+.