The effect of the heavy metal ions Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+ on permeability parameters of the nerve membrane was investigated. The ions were applied externally to single myelinated fibres of Xenopus laevis. The ionic currents, associated with potential steps were measured and analysed. 2. Zn2+ reversibly slowed down the kinetics of the K system. The effect at large potential steps was described as an increase of tau n. 3.4 mM-Zn2+ increased tau n about three times. 3. Ni2+ and Cu2+ also reversibly increased tau n in a similar way. The effect was larger than that of Zn2+. The other period 4 ions tested did not affect tau n markedly. Nor did the group 2b ion Cd2+. 4. The relative efficiency of the different ions agreed well with their tendency to form complexes with certain ligands. The effect was not a simple function of affinity for sulphydryl groups. 5. All the studied ions decreased PNa. The decrease was reversible except for that caused by Hg2+. Cu2+ caused the largest reversible effect. Hg2+ irreversibly decreased PNa at low concentrations (1-10 microM). 6. All ions studied shifted the Na activation curve in a positive direction along the potential axis. The effect was reversible except for that caused by Hg2+. The largest reversible shift was caused by Cu2+. 7. The relative efficiency of the different ions on the parameters studied in Xenopus axons showed great similarity to the corresponding efficiency on axons from other species, although some differences were noted.
