Upright posture induced a consistent increase in serum K with a blunted plasma aldosterone response in four male renal patients with selective hypoaldosteronism. Dietary Na restriction induced a dramatic increase in the suppressed plasma aldosterone without returning the elevated serum K levels to normal and abolishing the postural serum K change. Therefore it is concluded that "upright hyperkalemia" may not be caused exclusively by suppression or/and inadequate postural response of aldosterone. Cellular disturbance in the regulation of K distribution might be an additional explantation for the aldosterone unresponsive postural K change.