The in vitro neocortical brain slice technique was used to study electrophysiological properties of neurons from brain biopsies in 10 patients undergoing neurosurgical treatment for a variety of conditions, including focal epilepsy. The principal finding was the occurrence of orthodromically evoked depolarization shifts (DSs) and burst discharges in a proportion of neurons in slices from epileptogenic cortex. These evoked depolarizations and bursts had a number of properties in common with those from experimental epileptogenic foci in neocortex, including large amplitude and prolonged duration; long and variable latencies; and all or none, threshold type behavior, dependent on the parameters of orthodromic stimulation. Also DSs could not be evoked by intracellular stimulation, or blocked by hyperpolarizing current pulses once they had been orthodromically evoked. Responses of DSs to current thus differed markedly from those of neurons in epileptogenic guinea pig hippocampal slices. The results of these experiments suggest that intracellular events in human neurons involved in epileptogenesis are similar in appearance to those in various animal models. Neurons in chronic epileptogenesis are similar in appearance to those in various animal models. Neurons in chronic epileptogenic foci retain some of their abnormal properties within brain slices maintained in vitro.