1. Individual variations in the potassium content of the fresh muscles of frogs are notable even when computed as percentages of the dry solids. The potassium content averaged higher in freshly collected summer frogs than in winter frogs after a period of captivity.
2. Muscles show a loss of from 8 to 15 per cent of their potassium during perfusion with potassium-free Ringer solution but tenaciously hold the remainder.
3. Muscles, stimulated to contract under conditions that do not produce irreversible stages of fatigue, show losses of potassium no greater than those attributable to the presence of a potassium-free medium.
4. A condition favorable to the taking up of potassium probably occurs in a contracting muscle because rubidium and cesium, substances very similar to potassium in chemical and physiological behavior, are absorbed in retainable form by a contracting muscle but not by a resting one.