The purpose of the present study was to investigate the possible involvement of hippocampal NMDA (N-methyl-D-aspartate) receptors in spatial memory in rats by means of intrahippocampal injection of an NMDA antagonist, AP5 (D,L-2-amino-5-phosphonopentanoic acid). In Expt. 1, spontaneous motor activity (locomotor activity and rearing) was measured in an open-field after bilateral AP5 injection into the hippocampus, and it was shown that AP5 did not affect general activities within the dosage used in the present study. In Expt. 2, radial-maze performance was observed after the intrahippocampal injection of AP5. Rats were required to consume all the pellets which were put on each of the eight arms of the maze without revisiting the arms. AP5 produced dose-dependent impairment on the choice performance in the radial-arm maze, but did not have any effect on their running time. These results suggest that hippocampal NMDA receptors are involved in spatial memory.