Field and intracellular recordings were used to examine the effects of varying concentrations of penicillin on synchronous CA3 activity in guinea pig hippocampal slices. In addition to the high-amplitude bursts, extracellular recordings in the distal apical dendrites (700-1200 microns from the soma) revealed biphasic mini field potentials (MFPs) which were not evident at the soma in 2000 IU/ml. A long-lasting (76 ms) field potential (A potential) with a waveform similar to the positive component of the MFP initiated the bursts. The cellular correlate of the positive component of the MFP and of the A potential appeared to be an EPSP and that of the negative component of the MFP and IPSP. Reductions of penicillin concentration below 2000 IU/ml (3.4 mM) decreased the burst rate and amplitude and increased burst threshold. At concentrations below 250 IU/ml the bursts were blocked and the MFPs increased in amplitude and occurred rhythmically at a mean frequency of 2.6 Hz. At intermediate concentrations the bursts arose from the rhythmic background. This activity more closely resembles that recorded with electroencephalography in human epileptic foci than does the high-dose penicillin preparation and may provide a better model of epileptiform discharge.