This study has investigated the interactions between nitric oxide and haem protein radicals. The results demonstrate that nitric oxide interacts with activated ferrylmyoglobin species with reduction to metmyoglobin, but the extent and duration of the reduction depends on the relative concentrations of nitric oxide and hydrogen peroxide. Ferry1 myoglobin has a much greater relative potential for oxidising polyunsaturated fatty acid side chains in low density lipoproteins than in cell membranes. The peroxidative response can be modulated by nitric oxide: ferry1 myoglobin-mediated peroxidation of LDL may be enhanced or suppressed by nitric oxide depending on the relative concentrations of NO and hydrogen peroxide.