Increase of serotonin content and decrease in 5-hydroxyindolyl acetic acid (5-HIAA) were observed under hypoxic conditions (9,000 m, 3 hrs) which correlated with inhibition of monoamine oxidase A. Administration of the monoamine oxidase inhibitor pyrazidole caused the more pronounced increase of serotonin levels and decrease of 5-HIAA in control animals as well as in hypoxic animals. The catecholamines increased in hypoxia, pretreatment with pyrazidole prevented this increase. Preliminary administration of pyrazidole also increased the content of blood catecholamines, which reduced in hypoxia. Pyrazidole exhibited the antistress effect as the drug inhibited a decrease in adrenal ascorbic acid levels in hypoxic conditions.