This study demonstrates that transverse slices taken from the ventral hippocampus of adult rats perfused with a medium of normal ionic composition sustain spontaneous periodic field potentials due to the synchronous activity of a population of neurons. This ventral hippocampus spontaneous synchronous activity (VHSSA) in CA1 stratum pyramidale consisted of positive potentials (approximately 0.12 mV, 55 ms) occurring at a frequency of 2.8 +/- 0.2 Hz for hours without interruption. VHSSA was most frequently observed in slices taken 1-3 mm from the ventral end of hippocampus, and was absent in slices taken from tissue more than 4.5 mm away from it. Stimulation of Schaffer collaterals primed the appearance of potentials, which were similar to VHSSA and clearly distinguishable from excitatory postsynaptic potentials. In view of the known relative proneness of ventral hippocampus to epilepsy, we perfused ventral slices with high-[K(+)](o) medium (8 mM). Albeit reduced in amplitude, VHSSA persisted during the high-[K(+)](o) induced interictal-like epileptiform activity. We could not document any temporal relationship between the two phenomena. Low concentrations of the antagonist of gamma-amino-butyric acid receptors, type A, bicuculline (2-3 microM), which enhanced the high-[K(+)](o) induced epileptiform activity, reversibly blocked the VHSSA. We conclude that under standard in vitro conditions small circuits in the ventral hippocampus are most often and for long periods of time engaged in synchronous quasi-rhythmic low-frequency activity, generated locally by mechanisms substantially differing from those supporting epileptiform discharges.