Adrenochrome, a stable oxidation product formed after oxidation of adrenaline, strongly stimulates oxygen uptake occurring during the autoxidation of adrenaline, other catecholamines and ascorbate. Oxygen consumed is converted to hydrogen peroxide suggesting the occurrence of a redox cycling process. The reduction of adrenochrome operated by adrenaline is accelerated by the exclusion of oxygen indicating that the oxidation of adrenaline occurs directly and superoxide anion does not necessarily mediate it. Oxygen consumption, observed in the catecholamine/adrenochrome and ascorbate/adrenochrome systems, is due to the autoxidation of leucoadrenochrome that, at variance with adrenaline, easily autoxidizes also at physiological pH. Therefore, in these systems, leucoadrenochrome appears to be the major determinant of the production of superoxide anion.