Because myocyte depolarization and repolarization depend on intra- and extracellular shifts in ion gradients, abnormal serum electrolyte levels can have profound effects on cardiac conduction and the electrocardiogram (EKG). Changes in extracellular potassium, calcium, and magnesium levels can change myocyte membrane potential gradients and alter the cardiac action potential. These changes can result in incidental findings on the 12-lead EKG or precipitate potentially life-threatening dysrhythmias. We will review the major electrocardiographic findings associated with abnormalities of the major cationic contributors to cardiac conduction-potassium, calcium and magnesium.