The regulatory role of pith cells in the stem in Na(+) recirculation in sweet pepper was investigated by evaluating the transport characteristics of the plasma membrane of this cell type and comparison with those of root cells. Ion conductivity and Na(+) permeability of the plasma membranes of protoplasts of both cell types were studied with the patch-clamp technique in the whole-cell configuration, before and after addition of NaCl to the bath medium. Protoplasts of both pith and root cells showed outward rectifying currents with a reversal potential (V(r)) near to the equilibrium potential of K(+) (EK). Addition of NaCl to the bath medium caused a stronger shift of the reversal potential, V(r), in pith protoplasts than in root protoplasts, indicating that the outward rectified currents are permeable to Na(+), especially in the pith cells. After plant exposure to exogenous NaCl via the nutrient solution for 1 week, V(r) in the root cells was closer to EK than in the control plants and hardly shifted upon addition of Na(+). This indicated that the net permeability of the OR channel complement in the plasma membrane to Na(+) was lower following exposure to Na(+). V(r) in the pith protoplasts, on the other hand, shifted significantly more than in the control plants, suggesting an increase of the permeability to Na(+). Moreover, the Na(+) channel blocker amiloride blocked the currents in this cell type. It is concluded that pith cells have appropriate features of outward rectified currents to enable Na(+) accumulation or release when NaCl is present in or removed from the nutrient medium. Probably, exogenous NaCl even induced expression and formation of Na(+)-permeable channels in pith cells.