Although melatonin has been claimed to increase total antioxidant capacity (TAC) of blood serum, this does not seem probable in view of the low physiological concentrations of this hormone in blood, in relation to other antioxidants. This study demonstrates that enrichment of blood plasma with melatonin up to micromolar concentrations does not affect TAC in vitro thus excluding any significant direct contribution of melatonin to TAC of blood plasma.