Considerable evidence suggests that an N-methyl-D-aspartate (NMDA) receptor plays a crucial role in memory and cognitive function. To identify the role of this receptor in higher functions of the brain, we delivered antisense oligonucleotides against an NMDA-NR1 subunit (NR1) to the hippocampus in rats using the HVJ-liposome-mediated gene-transfer method. NR1 hippocampal knockdown was performed by the focal injection of the NR1 antisense-HVJ-liposome complex into the bilateral hippocampus. The blocking effect of NR1-antisense on the expression of NR1 was confirmed by Western blot analysis. Spatial memory was tested by a water maze task, and sensorimotor gating was examined by prepulse inhibition (PPI). Western blot analysis demonstrated that the NR1-antisense treatment specifically provided the down-regulation (about 30%) of NR1 protein levels in the hippocampus. The water maze task showed that the antisense treatment did not affect spatial memory, while the PPI test revealed that NR1 hippocampal knockdown caused a deficit in sensorimotor gating. We conclude that mild dysfunction of hippocampal NMDA receptor causes sensorimotor gating deficit and relatively intact in spatial memory.