Melatonin (N-acetyl-5-methoxytryptamine) is the chief secretory product of the pineal gland and synthesized enzymatically from serotonin (5-hydroxytryptamine). These indoleamine derivatives play an important role in the prevention of oxidative damage. In the present study, DMPD radical scavenging and cupric ion (Cu(2+)) reducing ability of melatonin and serotonin as trolox equivalent antioxidant activity (TEAC) was investigated. Melatonin and serotonin demonstrated 73.5 and 127.4 microg/mL trolox equivalent DMPD( radical+) scavenging activity at the concentration of 100 microg/mL. Also, at the same concentration, melatonin and serotonin showed 14.41 and 116.09 microg/mL trolox equivalent cupric ion (Cu(2+)) reducing ability. These results showed that melatonin and serotonin had marked DMPD(radical+) radical scavenging and cupric ions (Cu(2+)) reducing ability. Especially, serotonin had higher DMPD radical scavenging and cupric ions (Cu(2+)) reducing activity than melatonin because of its phenolic group.