The effects of a large number of neurotropic drugs have been attributed to changes in the metabolism of 5-hydroxytryptamine. The aromatic amino acid decarboxylase considered to decarboxylate both dihydroxyphenylalanine and 5-hydroxytryptophan requires pyridoxal phosphate as coenzyme. Thus, in pyridoxine deficiency one would expect a decrease of serotonin as well as the catecholamines of the brain. In the present study we have found a very significant decrease in brain serotonin of the pyridoxine-deficient growing rat. However, the brain levels of norepinephrine and dopamine were not altered. This decrease in serotonin does not result from a decrease either in the brain level of trytophan or the activity of tryptophan hydroxylase. Increased degradation of serotonin measured by the levels of its metabolite, 5-hydroxyindoleacetic acid is also excluded, thus suggesting the possibility that the decarboxylation of 5-hydroxytryptophan is decreased in pyridoxine deficiency.