The effect of K+ or Mg++ deficiency, or both, upon the electrocardiogram and plasma electrolytes has been studied in young pure-bred beagle dogs maintained on selected diets for eight weeks. K+ deficiency, in addition to resulting in the known electrocardiographic changes, caused a rise in serum Mg++ and hypokalemia. Mg++ deficiency resulted in a peaking of the T wave similar to that seen in hyperkalemia, although serum K+ was normal. In doubly deficient dogs diphasic T waves developed with marked terminal inversions. In two Mg++ deficient dogs, maintained on the diet for an additional seven months, hypokalemia developed as well as changes in the electrocardiogram indistinguishable from those seen in doubly deficient animals.