We previously reported that the levels of lipid hydroperoxides, one of oxidative stress markers, in the brain and peripheral organs such as liver, heart, and lung are significantly higher in senescence accelerated-prone 8 mice (SAMP8) than in their controls, senescence accelerated-resistant mice (SAMR1), at 3, 6, and/or 9 months of age. To ascertain the exact age at which the lipid hydroperoxide levels increase in SAMP8, we measured them in the brain and liver of SAMP8 and SAMR1 at both 1 and 2 months of age. At 1 month of age, there was no significant inter-strain difference in the levels in brain or liver. However, in SAMP8 both levels were significantly greater at 2 months of age than at 1 month of age, but no such difference was detected for SAMR1. The present results suggest that SAMP8 are exposed to elevated levels of oxidative stress from an early age (2 months old), and that this may be a cause of the senescence-related impairments and degeneration in the brain and peripheral tissues (such as liver, heart, and lung) seen in this strain.