Marked hyperkalemia was observed during and immediately after an infusion of arginine monohydrochloride in 2 patients with severe hepatic disease and moderate renal insufficiency. Both patients received brief courses of spironolactone before arginine treatment. In 1 of the patients, the hyperkalemia was associated with a fatal cardiac arrhythmia. Arginine shifted K from cells to the extracellular compartment, an effect directly related to its serum concentration. The profound hyperkalemia that occurred in these 2 patients is supposedly the result of an inability to metabolize the administered arginine and excrete the excess extracellular K. Caution is advised in administering arginine to patients with renal or hepatic insufficiency, or both.