Berberine is a benzyl tetra isoquinoline alkaloid which is widely used as an antimicrobial and an antidiarrhoeal. As berberine containing plants are virtually used in all forms of traditional medicine, our study aimed to examine the antioxidant activity of berberine using 2,2-diphenyl 1-picrylhydrazyl (DPPH) radical scavenging, nitric oxide scavenging, lipid peroxidation, superoxide scavenging, iron chelating activity and 2,2-azino bis(3-ethylbenzothiazoline-6- sulfonate) (ABTS) radical scavenging methods. The IC50 values for all the models were calculated by regression analysis. In all the models tested, berberine showed its ability to scavenge the free radicals in a concentration dependent manner. The present study thereby justifies the therapeutic potential of berberine. © 2006 Pharmaceutical Society of Japan.