Various lesion studies demonstrated that trace but not delay fear conditioning requires an intact hippocampal formation. Our present study examined the role of NMDA receptor activation and protein synthesis within the dorsal hippocampus for acquisition of fear memories following trace (5-s trace) and delay conditioning. To this end male C57BL/6JOlaHsd mice were chronically implanted with guide cannulae targeting the dorsal hippocampus. Fifteen minutes before conditioning mice received a bilateral intrahippocampal injection of either the NMDA receptor antagonist AP5 (0.5 or 1 microg per 0.5 microl per side) or of anisomycin, an inhibitor of protein synthesis (62.5 microg per 0.5 microl per side). Control mice were treated with vehicle (Ringer's solution). Blocking NMDA receptors before trace but not delay conditioning dose-dependently attenuated the freezing response to the tone as assessed 24 h after conditioning. The same findings were obtained after blocking protein synthesis within the dorsal hippocampus. These data indicate that the hippocampus shows synaptic plasticity during trace conditioning that requires an activation of NMDA receptors and protein synthesis as prerequisites for the acquisition of fear memory.