Abstract: Investigation of some isoquinoline alkaloids on monoamine metabolism in rat brain was conducted using HPLC-electrochemical detection. It was found that 1-tetrahydropalmatine (1-THP) and dl-tetrahydroberberine (dl-THB), 60 mg/kg ip, like dl-THP, a known short-term monoamine depletor, lowered monoamines and raised acidic metabolites (DOPAC, HVA and 5-HIAA) in the striatum and limbic area. But, the effect of dl-THP was much stronger than that of 1-THP. This may mean that d-THP is more active than 1-THP. The potency of dl-THB was found to be the weakest of the three alkaloids, dl-Tetrahydrocoptisine and dl-tetrahydrojatrorrhizine induced a slight reduction of dopamine only in the limbic area. However, both alkaloids markedly increased acidic motabolites in the striatum and limbic area. No significant changes on monoamines in rat brain was produced by berbamine, palmatine chloride, berberine chloride and 13-methylberberine iodide at doses of 60 mg/kg 30 min after intraperitoneal injection.