Mice of strains CC57/Br, A/He, C3H/He and CBA/Lac were studied at the age of 1, 2, 3, 4 and 5 months old. Activities of monoamine oxidases A and B (MAO A and MAO B) were increased in brain tissue of all the mice strain studied with ageing, although not simultaneously and dissimilarly. The ratio MAO A/MAO B approximated to 1.0 in old animals. At the age of 1 month old the MAO A/MAO B ratio was more than 1.0 in brain stem of mice of CC57/Br and C3H/He strains as well as in hemispheres of mice of A/He and CBA/Lac strains. In mice of A/He strain the MAO A/MAO B ratio exceeding 1.0 maintained within the first three months. Possible correlation between the elevated MAO A/MAO B ratio and high sexual activity of the A/He strain mice is discussed.