Low concentrations of penicillin (50-300 IU/ml) produce a pro-convulsant state in CA3 hippocampus characterized by low amplitude, low frequency (2-3 Hz) spontaneous field potential oscillations. Evoked field potentials were used to demonstrate that this distinct, coordinated behavior of the hippocampal neuronal network which is intermediate between normal activity and fully synchronized epileptiform bursting is the result of partial blockade of GABAA-mediated IPSPs. A new method for estimating the degree of IPSP block relative to convulsant doses of penicillin (2,000 IU/ml) indicates that the half-maximal effect occurs at 300 IU/ml. Fully synchronized bursts may require almost complete IPSP block.