t Theanine, r-glutamylethylamide, is one of the major amino acid components in green tea and many researchers have compared theanine's effects with glutamic acid because the chemical structure is similar. In the previous study, we demonstrated that theanine can pass brain-blood barrier and may play as an agonist or an antagonist of some receptors. In this study, we investigated the effects of theanine on neurotransmitter release in the rat brain striatum by in vivo brain microdialysis and examined whether theanine affected glutamate transporters by comparing it with a glutamate transporter blocker, L-trans-Pyrrolidine-2,4-dicarboxylic acid (L-trans-2,4-PDC). Because we investigated whether the effects of theanine is similar to L-trans-2,4-PDC on the brain neurotransmission, we measured dopamine release and some amino acids release which are known as excitatory or inhibitory neurotransmitters from neurons by theanine or L-trans-2,4-PDC perfusion into the rat brain striatum. L-trans-2,4-PDC or theanine perfusion into the brain striatum caused dopamine release from dopaminergic neurons. In addition, L-trans-2,4-PDC perfusion increased glutamic acid, aspartic acid and, whereas theanine perfusion prevented aspartic acid release and increased glycine release. These results suggested that the mechanism of dopamine release caused by theanine is different from glutamate transporter blockers or glutamic acid. Further, L-trans-2,4-PDC cause excitatory neurotransmission, whereas theanine may inhibit excitatory neurotransmission and cause inhibitory neurotransmission via glycine receptors.