Slow field potentials (SFP) lasting up to 5 s followed penicillin-induced bursts in CA 3 of guinea pig hippocampal slices and accounted for most of the interburst interval. The longest lasting component of the SFP which was maximal in the distal apical dendritic region was usually absent in s. pyramidale. Potassium flux underlies the entire potential, however, chloride-mediated components also contribute to the initial portion (approximately 1 s). The SFP was associated with neuronal afterhyperpolarization and glial cell depolarization, however, no unique cellular events could be identified with the 3 peaks of the SFP. The complex waveform of the SFP is probably the result of summation of neuronal and glial components.