A 'vectorial hypothesis' was proposed by which motor cortical cells possessing directional preference and directional spread could, as populations, generate movements in particular directions. The main assumptions of this hypothesis are that (a) cells exert a directional influence (vector) along the axis of their preferred direction, (b) this directional influence is on the same axis for all directions of movement, but (c) it is exerted toward the cell's preferred direction when there is an increase above the average discharge level, or toward the opposite direction when there is a decrease in discharge; and (d) for a particular movement direction, the vectorial components of individual cells sum linearly. Given these assumptions, the population vectorial sum was determined for each of the 8 direction of movement tested using experimentally observed data from 241 directionally tuned motor cortical cells. The direction of this population vector was within 11° from the straight-line movement direction. This was within the range of the directions of the movement trajectories produced by well trained animals. Possible relations between motor cortical cells and individual muscles or muscles synergies were discussed with respect to the hypothesis above. It was argued that the hypothesis would hold under either assumption, i.e. whether motor cortical cells are thought as controlling muscles or muscle synergies, assuming that no changes in the position of parts of the limb occur that would alter fundamentally the directions in which muscles exert their actions.