The motor cortical substrate associated with reaching was studied as monkeys moved their hands from a central position to one of eight targets spaced around a circle. Single-cell activity patterns were recorded in the proximal arm area of motor cortex during the task. In addition to the well-studied average directional selectivity ("preferred direction") of single-cell activity, we also found the time-varying speed of movement to be represented in the cortical activity. A single equation relating motor cortical discharge rate to these two parameters was developed. This equation, which has both independent (speed only) and interactive (speed and direction) components, described a large portion of the time-varying motor cortical activity during the task. Electromyographic activity from a number of upper arm muscles was recorded during this task. Muscle activity was also found to be directionally tuned; however, the distributions of preferred directions were found to be significantly different from cortical activity. In addition, the effect of speed on cortical and muscle activity was also found to be significantly different.