The acute effects of lithium on the central catecholamine and serotonin systems were investigated in well-defined cortical areas in the rat: the anterior cingulate cortex (CIN), the piriform-entorhinal region (PiEn), and the primary visual area (VIS) as well as in the hippocampus (HIP), the neostriatum (CPU; caudateputamen), and the olfactory bulbs (OBs). In these microdissected regions, the catecholamines noradrenaline (NA) and dopamine (DA), the indoleamine 5-hydroxytryptamine (5-HT; serotonin), as well as some of their major metabolites (3-methoxy-4-hydroxyphenylglycol; 3,4-dihydroxyphenylacetic acid; homovanillic acid; 3-methoxytyramine; 5-hydroxy-1-tryptophan; and 5-hydroxyindole-3-acetic acid) were assayed by using high-performance liquid chromatography (HPLC) with electrochemical detection. One hour after the administration of lithium chloride (2 and 10 mEq/kg; i.p.) the endogenous NA levels increased in the CIN and PiEn cortices, in the HIP, and in the CPU. The DA contents remained unchanged in the CPU, HIP, OB, and VIS cortex but were increased in the CIN and PiEn regions. These increases in cortical DA levels were accompanied by reductions in HVA and DOPAC. The levels of HVA and DOPAC but not 3-MT were also reduced in the CPU, in spite of a normal DA content. The discrepancies between changes of DA and the levels of its metabolites indicate changes in the turnover rates as well as an action of lithium on DA synthesis and/or storage in the nigrostriatal and mesocortical systems. The 5-HT contents were also increased by lithium throughout all regions, except for the OB.(ABSTRACT TRUNCATED AT 250 WORDS)