BRAIN metabolites were measured by proton magnetic resonance spectroscopy in five young (4–10 years of age) and six old (24–30 years of age) adult rhesus monkeys. The two age groups had similar levels of N-acetylaspar-tate and of choline relative to creatine, but the ratio of myo-inositol/creatine was higher in each old monkey than in any of the young animals. There was no significant relationship between the metabolite ratios and cognitive performance. The findings indicate that a consistent pattern of non-invasively detectable biochemical changes occurs in the brain with ageing. Whether these changes have functional significance in age-related pathologies, or are simply markers of brain ageing will be the subject of future studies.