Abstract: The aim of this study was to investigate the effect of aging and ovariectomy on various physiological parameters related to inflammation and oxidative stress in livers obtained from old female rats, and the influence of chronic administration of melatonin on these animals. Twenty-four female Wistar rats of 22 months of age were used. Animals were divided into four experimental groups: two intact groups that were untreated or given melatonin (1 mg/kg/day), and two ovariectomized groups that also untreated and treated with melatonin (1 mg/kg/day). After 10 wk of treatment, rats were sacrificed by decapitation, and livers were collected and homogenized. A group of 2-month-old female rats was used as young controls. Protein expression of inducible nitric oxide synthase (iNOS), heme oxygenase-1 (HO-1), IL-6, TNF-α and IL-1β were determined by Western blot analysis. The levels of nitric oxide metabolites (NOx), lipid hydroperoxide (LPO), TNF-α, IL-1β, IL-6 and IL-10 were determined. Levels of LPO in the liver homogenates as well as iNOS protein expression and NOx levels were increased in old rats as compared with young animals; this effect was more evident in ovariectomized animals. Pro-inflammatory cytokines TNF-α, IL-1β and IL-6 were significantly increased and anti-inflammatory IL-10 decreased during aging and after ovariectomy. Aging also significantly increased the expression of HO-1 protein, and ovariectomized rats showed an additional increase. Administration of melatonin, both to intact and to the ovariectomized animals significantly reduced NOx, LPO levels and pro-inflammatory cytokines in the liver as compared with untreated rats. Significant rice in IL-10 and reductions in the iNOS, HO-1, IL-6, TNF-α and IL-1β protein expression were also found in rats treated with melatonin. Oxidative stress and inflammation induced during aging in the liver are more marked in castrated than in intact females. Administration of melatonin reduces both these situations.