The dorsal hippocampus is required for trace fear conditioning, where the CS and US are separated in time, but not for delay fear conditioning, where the stimuli overlap. Despite evidence for the key role of glutamatergic NMDA receptors in a variety of forms of hippocampus-dependent memory, recent findings suggest that trace fear conditioning is only dependent on hippocampal NMDA receptor activity when the interval between CS offset and US onset is relatively long (i.e., >= 5 s Wanish et al. 2005, Misane et al. 2005, Chowdhury et al. SFN 2004). This suggests either that short-interval trace conditioning is not dependent on the dorsal hippocampus, or that it is dependent on NMDA receptor-independent mechanisms within the dorsal hippocampus. To investigate this issue further, the current experiments compared the effects of intrahippocampal infusion of the NMDA receptor antagonist AP5 and the AMPA receptor antagonist NBQX on fear conditioning with a 3-s trace interval. Fear-potentiated startle was used to mea...