How the intention to act results in movement is a fundamental question of brain organization. Recent work has shown that this operation involves the cooperative interaction of large neuronal populations. A population vector method, by transforming neuronal activity to the spatial domain, was used to visualize the motor cortical representation of the hand's trajectory made by rhesus monkeys as they drew spirals. Hand path was accurately reflected by a series of population vectors calculated throughout the task. A psychophysical rule relating speed to curvature, the "power law," was found in this cortical representation. The relative timing between each population vector and the corresponding portion of the movement was variable. The population vectors only preceded the movement in a predictive manner in portions of the spiral where the radius of curvature was greater than 6 centimeters. These results show that the movement trajectory is an important determinant of motor cortical activity and that this aspect of motor cortical activity may contribute only to discrete portions of the drawing movement.