The ability of tronchuda cabbage ( Brassica oleracea L. var. costata DC) to act as a scavenger of the reactive nitrogen species nitric oxide and peroxynitrite was investigated. The aqueous extracts obtained from tronchuda cabbage seeds and from its external and internal leaves exhibited a concentration dependent scavenging capacity. The antioxidant potential observed against the two reactive species was as follows: seeds > external leaves > internal leaves. In order to establish a possible correlation with the chemical composition of the extracts, the activity of ascorbic and sinapic acids and kaempferol 3- O-rutinoside was also studied. Among the compounds tested, sinapic acid showed the strongest antioxidant activity against both species.