It has been shown in our previous study that monoamine oxidase (MAO) activity in different brain regions are correlated with a microsomal oxidation phenotype. The data obtained in this study, using the microsomal oxidation inhibitor SKF525, and using animals with different duration of hexobarbital sleep, has shown that increased intensity of microsomal oxidation might be associated with increased MAO activity. Since the rats with short hexobarbital sleep time had higher content of hepatic microsomal cytochrome P450 than did rats with long hexobarbital sleep time. In addition, the rats with higher hepatic content of CYP450 had higher activities of MAO-A and MAO-B. Moreover, the microsomal oxidation inhibitor SKF-525 reduced brain and liver activities of MAOA and MAO-B. Consequently, MAO activities in a brain and a liver depend on the microsomal oxidation process.