TETRAHYDROPALMATINE, (+)- is an alkaloid found in the plants of the Carydalis species. Tetrahydropalmatine (THP) has two enantiomers with different effects on the brain dopaminergic system. Dopamine (DA) content reduction in the rat striatum induced by d-THP is much more dose-dependent. At a small dose, d-THP predominantly reduced the DA level but left serotonin and noradrenaline level unaffected. d-THP is probably a DA depletor. There were stereoselective differences between the two THP enantiomers on the activity of cytochrome P450 (CYP450) isozymes, i.e., d-THP had the potential to inhibit the activities of CYP2D6 and CYP1A2 isozymes, while l-THP inhibited CYP1A2 isozyme and induced CYP3A4 and CYP2C9 isozymes. The plasma levels of l-THP were always higher than those of d-THP in rats.