Sodium permeation into cortex cells of wheat roots was examined under conditions of high external NaCI and low Ca(2+). Two types of K(+) inward rectifier were observed in some cells. The time-dependent K(+) inward rectifier was Ca(2+)-sensitive, increasing in magnitude as external Ca(2+) was decreased from 10 mM to 0.1 mM, but did not show significant permeability to Na(+). However, the spiky inward rectifier showed significant Na+ permeation at Ca(2+) concentrations of 1 and 10 mM. In cells that initially did not show K(+) inward rectifier channels, fast and sometimes slowly activating whole-cell inward currents were induced at membrane potentials negative of zero with high external Na(+) and low Ca(2+) concentrations. With 1 mM Ca(2+) in the external solution, large inward currents were carried by Rb(+), Cs(+), K(+), Li(+), and Na(+). The permeability sequence shows that K(+), Rb(+) and Cs(+) are all more permeant than Na(+), which is about equally as permeant as Li(+). When some K(+) was present with high concentrations of Na(+) the inward currents were larger than with K(+) or Na(+) alone. About 60% of the inward current was reversibly blocked when the external Ca(2+) activity was increased from 0.03 mM to 2.7 mM (half inhibition at 0.31 mM Ca(2+) activity). Changes in the characteristics of the current noise indicated that increased Ca(2+) reduced the apparent single channel amplitude. In outside-out patches inward currents were observed at membrane potentials more positive than the equilibrium potentials for K(+) and Cl(-) when the external Na(+) concentration was high. These channels were difficult to analyse but three analysis methods yielded similar conductances of about 30 pS.