Previous studies of the motor cortex in behaving animals were focused on the relations between the activity of single cells, usually pyramidal tract neurons, and parameters of isometric contraction (e.g., intensity of force) or parameters of movement along one axis (e.g., flexion-extension) of a single joint (e.g., elbow or wrist). However, the commonly meaningful behavioral parameter is the trajectory of the hand in extrapersonal space, which is realized by simultaneous motions about two or three joints (e.g., elbow, shoulder, wrist) and concurrent engagement of several muscles. The spatial parameters of a straight trajectory are its direction and extent. We hypothesized that a major function of the motor cortex, among other possible roles, is the specification and control of the direction of the movement trajectory in space. This reference of motor cortical function to the control of spatial aspects of the trajectory differentiated our approach from the other approaches outlined above. We investigated the directional selectivity cells in the arm area of the motor cortex by recording their activity while monkeys moved their hands in various directions in space towards visual targets. There were two salient findings of these studies. First, the intensity of the discharge of single cells varies in an orderly fashion with the direction of movement in space, so that the discharge rate is highest with movements in a preferred direction, and decreases progressively with movements made in directions more and more away from the preferred one. Thus single cells are broadly turned around a preferred direction which differs among different cells. The second finding of our studies is a code by which this neuronal population can represent uniquely the direction of the trajectory in space. The outcome of this population code can be visualized as a vector in space that points in the direction of the movement well before the movement begins, and even in the absence of immediate movement. These results establish the motor cortex as a nodal point in the construction of motor signals controlling the direction of hand trajectory in space. Moreover, the population coding of direction may be of more general significance concerning the representation of information in neuronal ensembles and has been applied successfully to other areas of neurophysiological research.