Previous electrophysiological studies have demonstrated that in a subset of hippocampal slices from tissue resected from patients with mesial temporal lobe epilepsy, perforant path stimulation can elicit prolonged negative field-potential shifts in the dentate granule cell layer (Masukawa et al., 1989. Brain Res. 493, 168-174; Isokawa and Fried, 1996. Neuroscience 72, 31-37). In this investigation, hippocampal slices were prepared from rats: (1) 2-4 days following kainate treatment, when little or no reorganization of the mossy fibers would be present and (2) 3-13 months after kainate treatment, when mossy fiber reorganization would have occurred. In saline-treated controls, perforant path stimulation typically evoked a single population spike. In contrast, perforant path stimulation could evoke 3-12 population spikes in nearly all slices from kainate-injected rats 2-4 days and 3-13 months after treatment. The majority of slices from kainate-injected rats 3-13 months after treatment had qualitatively similar responses to perforant path stimulation as that observed in slices from kainate-injected rats 2-4 days after treatment. However, in 17% of the slices from kainate-treated rats 3-13 months after treatment (29% of rats), the multiple population spikes were followed by a prolonged negative field-potential shift (duration: 140 ms-1.5 s) with variable superimposed population spike activity. This type of epileptiform activity was only observed in slices with robust Timm's staining in the inner molecular layer and similar responses could also be evoked in these slices with hilar stimulation. Furthermore, pharmacological depression of inhibition by adding the GABA(A) receptor antagonist bicuculline unmasked hilar-evoked prolonged negative field-potential shifts in most slices from kainate-treated rats 3-13 months following treatment, and these slices had robust Timm's staining in the inner molecular layer. Such events were not observed in slices from saline-treated controls or kainate-injected rats 2-4 days after treatment. In conclusion, the prolonged negative field-potential shifts evoked to perforant path stimulation in normal ACSF were associated with mossy fiber reorganization, but the relative contribution of altered inhibition, increased synaptic excitation, or even non-synaptic mechanisms is unknown.