A comparative analysis of the effects of externally applied Rb+, Cs+, Sr2+, Ba2+and La3+on myelinated fibres under potential clamp conditions was made. The ions Cs+, Ba2+and La:l+(belonging to groups 1a, 2a and 3a of period 6 in the periodic system) were found to affect the K system in specific ways. Ba2+and La3+reduced the outward K+current associated with positive potential steps; the effect of La3+being larger than that of Ba2+. The effect of Ba2+depended on the K+concentration. Cs+, Ba2+and La3+reduced the inward K+tail current at repolarisation in solutions with high K+concentration. Cs+and Ba2+caused a rectification of the K+current, i.e. they reduced the inward tail current proportionally more than the outward steady state current. The effect of Baz+could not be described as a simple reduction of the permeability constant p¯K. Rb+was found to affect the time course of the K system by increasing the rate constant βn. Sr2*, Ba2+and La3+reduced the permeability constant p¯Na. These ions also shifted potential dependent parameters in general along the potential axis. The effect of La3+was larger than that of Sr2+and Ba2+. Cs2+, Ba2+and La3+reduced the p‐current, Ba2+and La3+the leak conduct