NO potently up-regulates vascular haem oxygenase-1 (HO-1), an inducible defensive protein that degrades haem to CO, iron and the antioxidant bilirubin. Since several pathological states are characterized by increased NO production and liberation of haem from haem-containing proteins, we examined how NO influences HO-1 induction mediated by haemin. Aortic endothelial cells treated with S-nitroso-N-acetylpenicillamine (SNAP), sodium nitroprusside (SNP) or diethylenetriamine-NONOate (DETA/NO) and haemin exhibited higher levels of haem oxygenase activity compared with cells exposed to NO donors or haemin alone. This was accompanied by a marked increase in bilirubin production and, notably, by a strong magnification of cellular haem uptake. A role for haem metabolites in modulating HO-1 expression by NO was assessed by exposing cells to SNAP, SNP or DETA/NO in medium derived from cells treated with haemin, which contained increased bilirubin levels. This treatment considerably potentiated HO-1 expression and haem oxygenase activity mediated by NO and the use of a haem oxygenase inhibitor abolished this effect. Both iron liberated during haem breakdown and the formation of nitroxyl anion from NO appeared to partially contribute to the amplifying phenomenon; in addition, medium from haemin-treated cells significantly augmented the release of NO by NO donors. Thus we have identified novel mechanisms related to the induction of HO-1 by NO indicating that the signalling actions of NO vary significantly in the presence of haem and haem metabolites, ultimately increasing the defensive abilities of the endothelium to counteract oxidative and nitrosative stress.