Cortical dysplasia, a disorder of neuronal migration, has a strong association with intractable epilepsy in humans but little is known about the physiologic abnormalities that are present in this condition. Fetal rats were exposed to external irradiation to experimentally produce diffuse cortical dysplasia. In vitro neocortical slices from adult irradiated and control animals were examined in physiologic solution and in the presence of the A-type gamma-amino butyric acid (GABAA) receptor antagonist, bicuculline methiodide. Epileptiform bursts were quantified by counting the number of negative field potentials per epileptiform event. In the presence of bicuculline, neocortical slices with cortical dysplasia demonstrated more robust epileptiform activity in the form of an increased number of negative field potentials per epileptiform event. This demonstrates that areas of experimentally induced cortical dysplasia possess an inherent hyperexcitability when GABAA-mediated inhibition is effectively blocked.