A widely accepted hypothesis is that long-term potentiation (LTP) is a synaptic mechanism of memory. NMDA receptors are critically involved in induction but not maintenance of LTP; therefore, their blockade should impair memory acquisition but not retrieval. In Experiment 1, we investigated the effect of a systemic NMDA receptor antagonist, CGP-40116 [D-isomer of CGP-37849: (E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (6 mg/kg, i.m.) 60 min before the testing session] on memory acquisiti...