The current method of localizing somatosensory and motor cortex during neurosurgical removal of abnormal tissue is Penfield's method of cortical stimulation. While useful, this method has drawbacks, in particular the need to operate under local anesthesia. Another method of localization, described here, involves intra-operative recording of short-latency somatosensory evoked potentials to stimulation of the contralateral median nerve, from electrodes placed directly on central cortex. Proper localization involves identification of potentials which invert in polarity across the central sulcus, identification of other potentials which are largest in the medial portion of the hand area of somatosensory cortex and do not polarity invert, and determination of the region of maximal potential amplitude. This method of localization works equally well whether the patient is under local or general anesthesia, but it occasionally fails in patients with tumors abutting or invading the hand area of sensorimotor cortex.