In 99 patients with mass lesions in and around the central region, the central sulcus was intraoperatively localized with the use of somatosensory evoked potential (SEP) phase reversal. In 33 of these patients, the motor cortex was directly stimulated and electromyographic responses were recorded from the forearm flexor, thenar, and hypothenar muscles. An additional 25 patients, with subcortical lesions or lesions directly located at the pyramidal tract, were continuously monitored during surgery by motor evoked potentials (MEPs). An exact determination of the central sulcus and tumor localization was possible in all patients; a phase reversal was obtained in 90.8% of the patients, and localization was possible as a result of anatomic structures and the loss of N20 or P20 of SEPs in the other 9.2%. MEPs were obtained in 32 of 33 patients and also in all 25 patients who underwent MEP monitoring at the beginning of tumor removal. From this study, it can be concluded that the combination of SEP phase reversal and modified electrical cortex stimulation is compatible with general anesthesia, although anesthesia was not systematically controlled according to a protocol. Although this study demonstrates that the combined SEP/MEP technique was feasible, it is not yet possible to demonstrate benefit in improving the outcome of patients. Concerning the safety of stimulation, the exact localization of the central sulcus by the noninvasive SEP method, compared with direct electrical stimulation, offered more safety for the patient. The modified technique of direct motor cortex stimulation needed much less charge density than did the common technique. The recording of electromyographic responses instead of movements made objective documentation possible, and the analysis of amplitudes and latencies will supply quantitative information about the motor system.