Intracerebroventricular injection of gamma-aminobutyric acid (GABA) was performed in male rats and the brain monoamines, 5-hydroxyindoleacetic acid (5-HIAA), tyrosine and tryptophan levels were measured. GABA induced within 30 min a marked dose-dependent increase in the brain contents of dopamine (DA), serotonin (5-HT), tyrosine and tryptophan, while noradrenaline (NA) was lowered. Large doses of GABA, i.e. 1.5-3 mg/rat, were required for these effects. Aminooxyacetic acid (AOAA), an inhibitor of GABA-transaminase, when given alone in a dose of 25 mg/kg i.p. caused a significant rise of DA, 5-HT and tryptophan. The combination of GABA and AOAA raised these levels more than either agent alone. Picrotoxin (4 mg/kg, i.p.) a claimed GABA receptor antagonist partially counteracted the GABA-induced DA rise. Monoamine synthesis was studied in different parts of the brain by measuring the accumulated dopa and 5-hydroxytryptophan (5-HTP), 30 min after NSD 1015 (3-hydroxybenzylhydrazine HCl, 100 mg/kg) an inhibitor of aromatic L-amino-acid decarboxylase, given i.p. 5 min after GABA. GABA caused a marked rise in dopa formation both in DA- and NA-predominated brain regions. Also 5-HTP formation was enhanced. The effects on both dopa and 5-HTP formation showed marked regional differences. The data suggest that GABA, by activating specific receptors, causes inhibition of firing of dopaminergic neurones and the opposite effect on the noradrenergic neurones. The nature of the effect on 5-HT metabolism needs further investigation.