Abstract: Mitochondrial DNAs (mtDNA) were isolated from brain tissue of three monkeys,I.e.,an old (over 12 years old) and a young (2-3 years old) rhesus monkeys,and a young (1-2 years old) crab-eating monkey.The method we used is alkaline lysis as described in our previous paper.Compared with marker DNA (λ/EcoRⅠ/Hind Ⅲ) on gel,the final yield was about 0.7 μg mtDNA/g brain tissue,a little lower than that from liver tissue.On the other hand,the homogenization of brain tissue is very easy;and the quality of the mtDNA is better than mtDNA from liver tissue,I.e.,there was much less protein and RNA in rough mtDNA from brain tissue,so further purification of the mtDNA by Sepharose-4B column or Rnase digestion is not needed.This method is thought to be useful in the study of mammal genetics.The mtDNAs from brain were compared with those from livers of the same monkey.When 16 restriction enzymes of 6 bp recognition were used,38-42 sites were observed in the samples.By combining the cleavage patterns for eachenzyme,we classified the three samples into three different restriction types.Meanwhile,the mtDANs from brains and livers of the same animals showed no difference.The results demonstrated further that mtDNA from different organs of the same animal had no difference.Our results about the old monkey also suggested that there might be no change in mtDNA sequences in the process of senility.