1. The effect of electroconvulsive shock (ECS) on extracellular concentration of dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) was examined with the use of in vivo microdialysis in rat striatum. 2. Extracellular concentration of DA was markedly increased up to 183% after single ECS, and that of DOPAC, HVA and 5-HIAA was also significantly increased. The increase after the eighth ECS was attenuated compared to their increase soon after the first ECS. After repeated ECS, baseline concentration of DOPAC, HVA and 5-HIAA was significantly increased, and baseline DA concentration tended to increase. 3. These results suggested that single and repeated ECS activated metabolism of DA and 5-hydroxytryptamine in rat striatum. Activated metabolism of DA may be responsible for the clinical effect of electroconvulsive therapy for parkinsonism.