Theanine, gamma-glutamylethylamide, is one of the major amino acid components in green tea. This study was undertaken to evaluate the effect of theanine intake on long-term potentiation (LTP) induction at hippocampal CA1 synapses and exposure to acute stress. Young rats were fed water containing 0.3% theanine after birth. Key findings: Serum corticosterone level was markedly decreased by theanine intake. Because this decrease can modify synaptic plasticity, the effect of theanine intake was examined focused on CA1 LTP induction. CA1 LTP induced by a 100-Hz tetanus for 1 s was almost the same extent in hippocampal slices from theanine-administered rats, whereas that induced by a 200-Hz tetanus for 1 s was significantly attenuated. 2-Amino-5-phosphonovalerate (APV), an N-methyl-d-aspartate (NMDA) receptor antagonist, significantly attenuated CA1 LTP induced by a 200-Hz tetanus in the control rats, but not in theanine-administered rats. Interestingly, APV completely blocked CA1 LTP induced by a 100-Hz tetanus in the control rats, while scarcely blocking it in theanine-administered rats. These results indicate that theanine intake reduces NMDA receptor-dependent CA1 LTP, while increasing NMDA receptor-independent CA1 LTP. Furthermore, neither 100-Hz tetanus-induced LTP nor 200-Hz tetanus-induced LTP was attenuated in theanine-administered rats after exposure to tail suspension stress, suggesting that the lack of NMDA receptor-dependent CA1 LTP by theanine intake is involved in ameliorating the attenuation of CA1 LTP after tail suspension. This study is the first to indicate that theanine intake modifies the mechanism of CA1 LTP induction.