The action of penicillin on synaptically evoked dendritic activity was examined with the use of hippocampal slice preparations. Orthodormic activation of CA1 pyramidal neurons produced an excitatory-inhibitory postsynaptic potential sequence recorded intracellularly in the dendrites. Treatment with penicillin resulted in the appearance of spontaneous and synaptically evoked multipeaked field potentials and associated depolarization shifts and spike burst generation in CA1 cells. Intracellular recordings revealed that penicillin produced no detectable change in passive membrane properties of the postsynaptic dendrites. However, the inhibitory postsynaptic potential was suppressed by penicillin, resulting in the release of intrinsic dendritic burst firing during synaptic activation. These findings emphasize the role of normal patterns of dendritic burst generation in the production of intense neuronal discharge during penicillin-induced epileptiform activities.