The cortical potentials preceding movement, negative slope (NS'), premovement positivity (PMP), and the initial slope of motor potential (MP), were studied in detail with a 29-channel averaged EEG mapping technique in normal subjects. Self-paced, voluntary movements of the right and left index fingers were performed up to 150 times, and topographic color maps were created from the averaged wave forms. The maps revealed NS' of the dominant hand on the vertex and NS' of the non-dominant hand on the contralateral centroparietal area. PMP appeared on the ipsilateral precentral area, and the initial slope of MP appeared on a small, distinct contralateral precentral area, presumably the hand motor area. The amplitudes of the potentials did not show significant differences between dominant and non-dominant hands. PMP and the initial slope of MP appeared significantly earlier preceding non-dominant hand movement as compared with dominant movement. The findings indicate some difference in cortical activity relating to dominant and non-dominant hand movement.