Dobutamine was administered in a dose of 10 ± 1 μg/kg/mm to 13 patients with severe idiopathic or ischemic dilated cardiomyopathy. Acute hemodynamic improvement was noted in all patients. All patients had a significant decrease in plasma potassium (4.6 ± 0.1 to 4.2 ± 0.2 mEq/liter, p < 0.0001) at peak infusion. The decrease in potassium persisted for at least 45 minutes after discontinuing the infusion. Three patients had exacerba tion of baseline ventricular arrhythmias that resolved with infusion discontinuation. Changes in plasma norepinephrine could not explain the potassium decrease or arrhythmia production, which also significantly decreased in these patients (771 ± 123 to 524 ± 73 pg/ml, p < 0.01). It is concluded that dobutamine causes a significant decrease in plasma potassium and that the decrease persists at least 45 minutes after the infusion is discontinued.