One of the enigmatic properties of nitric oxide has been the findings that nitric oxide can exhibit both anti-inflammatory and pro-inflammatory activities. While this has usually been explained in terms of the enzyme source of NO (constitutive vs inducible) it remains that NO is a poorly reactive free radical which is inconsistent with its role as a cytotoxic agent. We have proposed that the conversion of NO to other more reactive species (N2O3, NO2, and ONOO-) accounts for the pro-inflammatory effects of NO. To examine this hypothesis further, we addressed the ability of antioxidants with known anti-inflammatory properties, to modify the degradation of NO or peroxynitrite (ONOO-). NO levels were monitored continuously with a microelectrode electrochemical method developed in house. Peroxynitrite levels were determined spectrophotometrically. The antioxidants chosen were: ascorbic acid, 5-aminosalicylic acid (5-ASA), histidine, homocysteine and three natural antioxidant preparations containing various proanthocyanadins, isoflavones and alkaloids (Phytolens, una de gato and sangre de grado). Cytotoxicity to peroxynitrite was determined in cultured human colonic epithelial cells. In terms of oxidative NO degradation, NO degradation was promoted by the three nutraceutical preparations and homocysteine. However, 5-ASA and histidine had no effect on NO whereas ascorbic acid retarded the oxidative degradation of NO. In contrast, peroxynitrite was rapidly degraded and its cytotoxicity attenuated by these antioxidants, including ascorbic acid. Thus, we conclude that antioxidants do not predictably affect NO levels. The modification of NO-dependent pro-inflammatory mechanisms by antioxidants is more likely to involve interaction with nitrogen oxides generated downstream from NO rather than with NO directly. Targeting nitrogen oxides other than nitric oxide may be a fruitful approach to novel therapeutics.