Theanine, γ-glutamylethylamide, is one of the major amino acid components in green tea and is synthesized from ethylamine and glutamate in green tea leaves. We reported the effect of theanine intake after weaning on stress-induced impairments of long-term potentiation (LTP) and recognition memory; hippocampal CA1 LTP and 24-h object recognition memory were impaired in the control rats after exposure to stress, but not in theanine-administered rats. NMDA receptor-independent CA1 LTP component was increased after theanine intake and the increase may be involved in the rescue of their impairments, because NMDA receptor-independent CA1 LTP is less susceptible to stress than NMDA receptor-dependent CA1 LTP. In the present study, the advantageous effect of theanine intake on the retention of object recognition memory was examined in young rats, which were fed water containing 0.3% theanine for 3 weeks after weaning. The rats were subjected to the object recognition test in the same manner. Object recognition memory was maintained in the control rats 24 h after training, but not 48 h after training. In contrast, object recognition memory was maintained 48 h after training in theanine-administered rats. When in vivo dentate gyrus LTP was induced by delivery of high-frequency stimulation (10 trains of 20 pulses at 200 Hz separated by 1 sec), it was more greatly induced in theanine-administered rats than in the control rats. The levels of brain-derived neurotropic factor and nerve growth factor in the hippocampus were significantly higher in theanine-administered rats than in the control rats. The present study indicates the advantageous effect of theanine intake after weaning on the retention of recognition memory. It is likely that theanine intake is of advantage to the modulation of hippocampal function after weaning.