Hepatocytes exhibit a non-specific immune response by expressing the enzyme inducible nitric oxide (NO) synthase (iNOS, NOS2) through the stimulation of a mixture of cytokines, or a single cytokine such as interleukin-1beta. We examined the age-dependent inducibility of the iNOS gene expression and the capacity of NO production in response to lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) in primary cultured rat hepatocytes that were isolated from the livers of rats, 3 (young) and 24 (aging) months of age. NO production (NO2-), indicating iNOS activity, was much higher in the young rat hepatocytes following stimulation with LPS or IL-1beta. Likewise, in the young hepatocytes, Western blot analyses showed much higher protein levels in the iNOS expression; it was also a little higher in mRNA levels that were analyzed by RT-PCR. Furthermore, after stimulation with IL-1beta, the levels of transactivation of nuclear factor-KB (NF-kappaB) that were involved in the induction of the iNOS gene were reduced without a significant difference in the aged cells. Therefore, the decrease of NO formation in the aged hepatocytes was due to the belated and incomplete inducibility of the iNOS protein expression, together with a minor contribution of the reduced-transactivation of NF-kappaB. These results suggest that the age-related decline of the iNOS gene expression in primary rat hepatocytes may be associated with the increased incidence of many infective diseases with aging.