Morphological data from humans with temporal lobe epilepsy and from animal models of epilepsy suggest that seizure-induced damage to dentate hilar neurons causes granule cells to sprout new axon collaterals that innervate other granule cells. This aberrant projection has been suggested to be an anatomical substrate for epileptogenesis. This hypothesis was tested in the present study with intra- and extracellular recordings from granule cells in hippocampal slices removed from rats 1-4 months after kainate treatment. In this animal model, hippocampal cell loss leads to sprouting of mossy fiber axons from the granule cells into the inner molecular layer of the dentate gyrus. Unexpectedly, when slices with mossy fiber sprouting were examined in normal medium, extracellular stimulation of the hilus or perforant path evoked relatively normal responses. However, in the presence of the GABAA-receptor antagonist, bicuculline, low-intensity hilar stimulation evoked delayed bursts of action potentials in about one-quarter of the slices. In one-third of the bicuculline-treated slices with mossy fiber sprouting, spontaneous bursts of synchronous spikes were superimposed on slow negative field potentials. Slices from normal rats or kainate-treated rats without mossy fiber sprouting never showed delayed bursts to weak hilar stimulation or spontaneous bursts in bicuculline. These data suggest that new local excitatory circuits may be suppressed normally, and then emerge functionally when synaptic inhibition is blocked. Therefore, after repeated seizures and excitotoxic damage in the hippocampus, synaptic reorganization of the mossy fibers is consistently associated with normal responses; however, in some preparations, the mossy fibers may form functional recurrent excitatory connections, but synaptic inhibition appears to mask these potentially epileptogenic alterations.