1. Dopamine, norepinephrine, serotonin and 5-hydroxyindol-acetic acid levels were studied in discrete brain areas of four wild rodent species and compared with those of laboratory mouse. 2. Monoamine levels showed significant differences in all the species studied. 3. Although it is not possible to exclude the changes induced by the captivity's stress, the results suggest that the natural hypoxic environment (high altitude) and/or the natural diet play an important role on the activity of the monoaminergic neurons. 4. Species which showed a better behavioral pattern of adaptation also showed significant differences in brain monoamine levels.