A chromatographic method with electrochemical detection was used to measure noradrenaline (NA), dopamine (DA), and serotonin (5-HT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3-MT) and 5-hydroxyindoleacetic acid (5-HIAA) in several brain areas (hypothalamus, hippocampus, striatum and rest of the brain) of rats exposed to a 7000 m simulated altitude for 3 hr. This new direct approach, not using a pharmacological tool, provides further information on the hypobaric hypoxia effects on the main DA and 5-HT metabolic steps. In the hypothalamus, a decreased NA level with increased DA and DOPAC contents was considered as the result of dopamine-beta-hydroxylase activity impairment. The decrease of both 5-HIAA and DOPAC in all brain areas provides further evidence of the hypoxia-induced decrement in MAO activity. Furthermore, in the striatum, it was shown that catechol-O-methyl transferase, up to now considered unaffected by hypoxia, may also be altered by oxygen deficiency.