The effects of hypo- and hyperthyroidism were investigated on brain levels and accumulation rates (after pargyline) of 5-hydroxytryptamine (5-HT), norepinephrine (NE) and dopamine (DA) in discrete brain regions of the adult rat. Whereas NE remained unchanged in all brain areas except in the cerebellum, alterations in brain 5-HT and DA suggest that the behavioral abnormalities associated with thyroid dysfunction in adulthood may be related to neurotransmission disturbances. In hypothyroidism, 5-HT content decreased in cerebral hemispheres and mesodiencephalon and DA content decreased in these regions and also in cerebellum and pons-medulla. Concomitantly, accumulation rate of 5-HT was lower in pons-medulla whereas that of DA was increased in cerebral hemispheres and mesodiencephalon. In hyperthyroidism, 5-HT levels increased in cerebral hemispheres alone. Accumulation rate of 5-HT increased in pons-medulla and that of DA increased in mesodiencephalon. These data indicate that the influence of thyroid hormones on monoamines (MAs) in the adult brain varies with the neurotransmitter and the brain area considered.