During diuretic treatment alterations in myocardial cellular excitability due to potassium-induced disturbances of the membrane potential may arise without obvious changes of total body K. This may be explained by coexisting disturbances of the acid/base balance and other ions such as magnesium, two factors which independently influence the transport of potassium across the cell membrane. The consequence may be cardiac arrhythmias, particularly in the presence of digitalis. On the other hand hypokalemia, induced by diuretics, may also be accompanied by a significant depletion of total body K, bringing about more general consequences. It must be considered essential to maintain a normal general electrolyte balance during diuretic therapy. Potassium supplements may be used if true depletion of total body K is suspected, e.g. in hypokalemia with acidosis. Its use may, however, otherwise be questioned, as it is only directed towards the potassium situation and neglects the influence of diuretics upon other ions, such as H+ and Mg++. The potassium-sparing agents amiloride and triamterene normalize the general electrolyte situation in the distal tubules and should thus be regarded as drugs of first choice. Spironolactone has identical properties concerning the electrolytes but more serious side effects. It should be preferred when a significant secondary hyperaldosteronism is suspected and/or a more intense diuresis or an accentuated antihypertensive action is desired. The role of captopril in this context has not yet been established.