The effect of lithium and carbamazepine in the treatment of bipolar affective disorder is well established. Although a number of biochemical effects have been found, the exact molecular mechanisms underlying their therapeutic actions have not been elucidated nor are the target regions in the brain identified. Taken into account the important role of the cyclic AMP second messenger system in the regulation of neuronal exitability and the indications of its involvement in the patophysiology of bipolar affective disorder, we have focused on the drug effects on cyclic AMP levels. The objectives of this investigation were to measure the effects on basal cyclic AMP levels, and to locate target regions within the rat brain after long-term administration of lithium and carbamazepine. Drug treatments were carried out for a period of 28 days. After either drug treatment the cyclic AMP level was increased 3-4 times in frontal cortex but unchanged in hippocampus, hypothalamus, thalamus, amygdala and in cerebellum. In neostriatum the cyclic AMP level was decreased to about 30% after treatment with lithium. We suggest the common region-selective effect, observed for both drugs in frontal cortex, to be essential for the therapeutic actions of lithium and carbamazepine.