Seventeen elderly patients are described, who showed minor changes in the e.c.g. recovery phase. Serum potassium levels appeared to be normal, but red-blood-cell potassium was found to be more or less markedly reduced. Restoration of the latter to normal values, which followed treatment with potassium salts was also associated with normalization of the e.c.g. pattern. These observations demonstrate that electrolyte disorders, and particularly hypokalaemia, may determine 'minor' alterations of repolarization much more frequently than is usually thought, and that determination of serum potassium levels is not a reliable tool for detecting potassium depletion, which is more accurately reflected by red-blood-cell potassium concentration.