We have recorded movement-related cortical potentials (MRCPs) to voluntary middle finger extension from 10 young and 10 old subjects free of neurological disease using the method of detecting EMG onset associated with each movement described by Barrett et al. (1985). The slow potential shifts preceding movement were measured by fitting a linear regression line to the wave forms to obtain a measure of their slope. Three separate potential shifts were identified. The first had a scalp distribution and onset latency similar to the Bereitschaftspotential (BP) first reported by Kornhuber and Deecke (1964, 1965). The potential shift immediately preceding movement corresponded with the NS' of Shibasaki et al. (1980). We identified, for the first time, a third shift intervening between BP and NS' and named it the intermediate shift (IS). The onset of BP occurred about 1.6 sec before EMG onset and was followed by IS which began about 875 msec before movement. The onset of NS' occurred 300 msec before EMG onset and terminated about 90 msec before this event. The slope of BP preceding right finger movement was steeper than that preceding left hand movement in all our right-handed subjects. The distribution of BP was symmetric about the midline. The IS potential shift had a slope which was steeper on the average preceding left finger movement than right. The distribution of IS was symmetric about the midline preceding left finger movement but had a contralateral tendency preceding right hand movement. NS' had a maximum slope at contralateral electrodes over the hand motor area and parietal areas. It was suggested that the BP potential shift originates in the supplementary motor area on the medial surface of the cerebral cortex. The differing distribution of the IS shift for the two hands suggests that this potential may be generated bilaterally preceding left finger movement but from the contralateral hemisphere only preceding movement of the right finger. The most likely origin of this potential was thought to be superior premotor cortex. NS' was considered to originate in primary motor cortex with possible contributions from other cortical areas associated with movement.