Lithium remains the most widely used therapeutic agent for bipolar affective disorder, particularly mania. Although many investigators have studied the effects of lithium on abnormalities in monoamine neurotransmitter as a pathophysiological basis of affective disorder, the action mechanism of lithium ion remains still unknown. To explore the action mechanism of lithium in the brain, we examined the effects of lithium on the extrasynaptosomal concentrations of catecholamines and their metabolites. Synaptosomes were prepared from the rat forebrains and assays of catecholamines and metabolites were made using HPLC with an electrochemical detector. Lithium of 1mM decreased the extrasynaptosomal concentrations of NE from the control group of 3.07⁑1.19 to the treated group of 0.00⁑0.00 (ng/ml of synaptosomal suspension) but not that of DHPG. It can be suggested that lithium increases synaptosomal uptake of NE. Increased intraneuronal uptake of NE would decrease neurotransmission and extraneuronal metabolism of NE. Because increased brain NE metabolism and neurotransmission have been suggested as important components in the pathophysiology of bipolar affective disorder, especially mania, lithium-induced increase of intraneuronal NE uptake can be suspected as an action mechanism of therapeutic effect of lithium in manic patient, possibly in bipolar affective disorder.