The effect of divalent cations and of high chloride ion concentration on voltage-clamp currents in the plasmalemma of C. australis was investigated. Either calcium or strontium ions were necessary in the medium for a transient current to appear during a voltage clamp. The transient current with strontium was about 20% of that with calcium present. Barium, cadmium, magnesium, manganous, or nickelous ions were unable to replace calcium in this function. In media containing a chloride concentration of 30 m. It was concluded that calcium ions mediate an increase in the permeability of the plasmalemma not to chloride specifically but to anions such as Cl-, Br-, and NO- 3, following a suitable stimulus. The same peak anion permeabilities were reached and the time courses of the permeability changes were similar during action potentials and during voltage clamps.