1. The development of epileptiform discharges in response to tetanic stimulation of the Schaeffer collaterals was studied by using extracellular field potential recordings in CA1, CA3, dentate gyrus, and entorhinal cortex and intracellular recordings in CA1 neurons in rat hippocampal-parahippocampal slices, which were cut so as to maintain reciprocal connections between entorhinal cortex and hippocampus in vitro. 2. The first type of epileptiform discharge to develop was an immediate afterdischarge, which grew in duration and amplitude with repeated stimulation trains at 10-min intervals, until it plateaued after five to nine trains at 40-s duration, on average. This afterdischarge, when fully developed, consisted of an early, high frequency tonic component, followed by a later, lower frequency clonic component. Fully developed primary afterdischarges were all-or-none, in that they had a definite threshold, and varied little in amplitude or duration when activated by threshold or suprathreshold stimulation. The primary discharge could be recorded simultaneously throughout the hippocampal-parahippocampal slice, providing evidence for the intact reciprocal connections between hippocampus and entorhinal cortex. Intracellular recordings in CA1 neurons revealed that during the tonic phase of the afterdischarge, neurons were depolarized by 15-30 mV and gradually repolarized during the clonic component. 3. After full development of the primary afterdischarge, a delayed secondary epileptiform discharge began to appear after five to nine stimulation trains. This late discharge began 2-5 min after the stimulation train and progressed in amplitude and duration with repeated stimulation, in some cases to 2-3 h long self-sustained epileptiform discharges. Like the primary afterdischarge, the secondary discharge could be recorded simultaneously throughout the hippocampal-parahippocampal slice, and individual bursts comprising the secondary discharge occurred at earliest latency in the dentate gyrus, followed by activation in CA3, CA1, and finally in the entorhinal cortex. Intracellular recordings in CA1 neurons established that the secondary discharge occurred without an accompanying depolarization. Rather, it appeared as synaptic bursts developing in an escalating frequency barrage, initiated 2-5 min after the primary afterdischarge. 4. Lesioning studies were conducted to begin determining the site of origin of the secondary epileptiform discharge. After appearance of the secondary discharge, the mossy fibers were cut. This lesion abolished the secondary discharge but did not block the primary afterdischarge. Moving the stimulating electrodes from the Schaeffer collaterals to the mossy fibers proximal to the cut reestablished a truncated secondary discharge. In a second lesioning experiment, a cut was made through the subicular region of the hippocampal-parahippocampal slice before the onset of stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)