The effects of single acute hypoxia (9000 m above the sea level, 3h) and intermittent hypoxia (5000 m 4 h daily during 3 and 10 days) were studied. Acute hypoxia was characterized by the increase in the content of brain serotonin, dopamine and norepinephrine accompanied by a decrease of deaminated product. The increase of monoamine contents in hypothalamus and adrenals during initial stage caused to considerable extent by their deamination and the monoamine decrease under 10-day hypoxia were typical for intermittent hypoxia. The administration of MAO a inhibitor pyrazidol promoted the increase in brain serotonin content, normalized brain catecholamine contents and demonstrated positive effect on the animal state.