Design: Adult patients who are candidates for standard surgical care for medically intractable epilepsy will undergo standard noninvasive evaluation to identify the location of the seizure focus. After surface EEG confirms the side and approximate location of the seizure focus, a depth electrode with a central lumen will be placed in the seizure focus for the purpose of this research study. The lumen in the depth electrode will be used to deliver vehicle (artificial cerebrospinal fluid) or muscimol, a neurotransmitter (gamma-aminobutyric acid, GABA) receptor agonist that suppresses neuronal activity. During the infusion trials, patients will be monitored by neurologic examination and by video-EEG. Following the trials, the epileptic focus will be resected, which is standard care for medically intractable epilepsy, and the surgical specimen will be examined for any histological changes caused by the infusions.