Progressive ageing is associated with an increment of biomolecules modified through oxidation as a result of the action of free radicals deriving from reactive oxygen species that attack biomolecules. During ageing many alterations of renal functions have been reported. Renal ageing is associated with a progressive decline of glomerular filtration, renal blood flow and augmented vascular resistance. The kidney is a very important source of inducible nitric oxide synthase (iNOS) in both epithelial and vascular structures. In this study we have investigated mRNA and protein iNOS expression and localization and nitric oxide (NO) production in young and aged rats. An increased expression of iNOS occurs in rat kidney during ageing. In the aged rat, an increase in the values of both iNOS-RNA and iNOS protein was observed through rtPCR and Western blot analysis. The activities of three isoforms of NOS were also seen. In the aged rat kidney the production of NO decreased, due to the reduction of the activities of the three NOS. This suggests that in the aged rat a progressive increase of superoxide anion does not imply an increase in the production of NO which functions as a scavenger molecule, causing oxidative stress with accumulation of reactive oxygen species (ROS).