Experiments were performed on knockout Tg8 mice lacking monoamine oxidase A gene that plays a major role in dopamine catabolism. The study by the method of high-performance liquid chromatography revealed considerable regional differences in the contents of dopamine and its metabolite dihydroxyphenylacetic acid in brain structures of these animals. Tg8 mice differed from the parent C3H/HeJ strain by low level of dihydroxyphenylacetic acid in the striatum, midbrain, hypothalamus, and hippocampus and high concentration of dopamine in the striatum. No differences were revealed in the contents of dopamine and dihydroxyphenylacetic acid in the frontal cortex and amygdala. The 2.4-4.8-fold decrease in the content of dihydroxyphenylacetic acid in various brain structures was not accompanied by changes in dopamine concentration. These data reflect the effective compensation for deficiency of dopamine metabolism. Our results suggest that monoamine oxidases A and B and catechol-O-methyltransferase play different roles in dopamine metabolism in various brain structures.