In humans with temporal lobe epilepsy and kainate-treated rats, the mossy fibers of the dentate granule cells send collateral axons into the inner molecular layer. Prior investigations on kainate-treated rats demonstrated that abnormal hilar-evoked events can occasionally be observed in slices with mossy fiber sprouting when gamma-aminobutyric acid-A (GABAA)-mediated inhibition is blocked with bicuculline. However, these abnormalities were observed infrequently, and it was unknown whether these rats were epileptic. Wuarin and Dudek reported that in slices from kainate-induced epileptic rats (3-13 mo after treatment), hilar stimulation evoked abnormal events in most slices with mossy fiber sprouting exposed simultaneously to bicuculline and elevated extracellular potassium concentration [K+]o. Using the same rats, extracellular recordings were obtained from granule cells in hippocampal slices to determine whether 1) hilar stimulation could evoke abnormal events in slices with sprouting in normal artificial cerebrospinal fluid (ACSF), 2) adding only bicuculline could unmask hilar-evoked abnormalities and glutamate-receptor antagonists could block these events, and 3) increasing only [K+]o could unmask these abnormalities. In normal ACSF, hilar stimulation evoked abnormal field potentials in 27% of slices with sprouting versus controls without sprouting (i.e., saline-treated or only 2-4 days after kainate treatment). In bicuculline (10 microM) alone, hilar stimulation triggered prolonged field potentials in 84% of slices with sprouting, but not in slices from the two control groups. Addition of the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5), either blocked the bursts or reduced their probability of occurrence. The alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), always eliminated the epileptiform bursts. In kainate-treated rats with sprouting, but not in saline-treated controls, abnormal hilar-evoked responses were also revealed in 6-9 mM [K+]o. Additionally, 63% of slices with sprouting generated spontaneous bursts lasting 1-40 s in ACSF containing 9 mm [K+]o; similar bursts were not observed in controls. These results indicate that 1) mossy fiber sprouting is associated with new glutamatergic pathways, and although NMDA receptors are important for propagation through these circuits, AMPA receptor activation is crucial, 2) modest elevations of [K+]o, in a range that would have relatively little effect on granule cells, can unmask these new excitatory circuits and generate epileptiform bursts, and 3) this new circuitry underlies an increased electrographic seizure susceptibility when inhibition is depressed or membrane excitability is increased.