We examined the effect of daily melatonin supplementation on liver apoptosis induced by aging in rats. Young (3-month-old) and aged (24-month-old) Wistar rats were supplemented daily with melatonin in their drinking water (20 mg/L) for 4 weeks. Aged rats showed increases in the liver concentration of thiobarbituric acid-reactive substances and in the oxidized/reduced glutathione ratio. These increases were accompanied by apoptotic ultrastructural alterations and increases in cytochrome c mitochondrial release, Bax to Bcl-2 relative expression, and activity of caspase-3. No significant changes were observed in Fas-ligand (Fas-L) expression and caspase-8 activity. Melatonin administration was able to abrogate changes detected in aged rats. Data suggest that liver apoptotic cell death is induced by reactive oxygen species, via the intrinsic signalling pathway, and that the antiapoptotic action provided by melatonin is related to its antioxidant effect, with reduction of cytochrome c release by the modulation of Bcl-2 and Bax genes.