The effect of external Ca, Mg, Sr, Ba and La concentration was studied in potential clamp experiments on single myelinated nerve fibres of the frog (Xenopus laevis). An increase in cationic concentration caused a positive shift of the Na permeability (PNa) curve along the potential axis, a decrease in max.PNaand a decrease in the slope of thePNacurve. The effectiveness of Mg, Sr and Ba was similar and about 0.6 compared to that of Ca, whereas La was about 80 times more effective than Ca.