The EL/Suz (EL) mouse is a strain that is highly susceptible to convulsive seizures after repeated sensory stimulation. Its control strain, DDY/Jc1 (DDY), is less susceptible under similar conditions. The seizure prone phenotype is the result of differences at several genetic loci. In vivo electrical recordings from the seizure prone EL mouse brain have shown that the appearance of abnormal discharges in the hippocampus are critical to the onset of generalized seizures, indicating that the hippocampus plays an important role in EL mouse seizure activity. In the present study, electrophysiological differences between EL and DDY mice (9-15 weeks of age) were examined by comparing field potentials recorded from the dentate granule cell layer of hippocampal brain slices from mice that had not been stimulated to induce seizures. In control physiological solution, no significant differences were observed in characteristics of perforant path evoked field potentials or in paired pulse depression of evoked field potentials using 20 to 300 ms interstimulus intervals. After 60 min of disinhibition following bicuculline (10 microM) exposure, however, prolonged large amplitude potentials, paroxysmal discharges, were evoked by perforant path stimulation in the dentate gyrus of EL mice but were absent in the DDY strain. Paroxysmal discharges were curtailed by APV and were similar to responses recorded from the dentate gyrus in hippocampal brain slices from temporal lobe epileptic patients. The field response to hilar stimulation was identical in both strains and was composed of a single population spike before and after bicuculline exposure. Mossy fiber terminals were not present in the molecular layer of either strain. We propose that the mechanisms leading to a greater likelihood of paroxysmal discharge generation in EL mouse may be important in the development and/or generation of epileptic seizures in this mouse strain and may be a significant phenotypic difference between the EL mouse and its parent strain.