The lumbar spinal cord of rats and cats has been used as a model system to study the epileptogenic effects of penicillin. Within a few minutes after the application of penicillin, segmental myoclonus develops. Intracellular recording from cat motoneurons demonstrates that penicillin induces periodic large spontaneously occurring depolarizing synaptic potentials. Either these responses or directly applied intracellular current pulses evoked prolonged depolarization of the motoneuron soma, which represent an alteration in membrane properties of motoneurons. The prolonged depolarization has a threshold for termination by a hyperpolarizing current.