Model rats of methyl mercury intoxication were made by orally administering 5 mg mercury/kg methyl mercury daily for 12 days. Proton magnetic resonance imaging and phosphorus-31 nuclear magnetic resonance spectroscopy measurements were performed on the brain of these model animals in vivo under anesthesia. Proton images contrasted with the longitudinal or transverse relaxation times of protons on water and lipid molecules exhibited an internal structure of the poisoned brain. No particular difference was, however, observed between the poisoned and normal control rats in either image. On the other hand phosphorus-31 NMR spectra showed a 17% decrease in phosphocreatine and a corresponding increase in inorganic phosphate in the methyl mercury-poisoned brain. It was also shown that the ATP concentration and the intracellular pH were maintained at a normal level even in the poisoned brain.