1. We lesioned the contralateral arm area of the primary motor cortex (M1) in a monkey that had been trained to make rapid step-tracking movements of the wrist in different directions. We examined movement kinematics and electromyographic (EMG) activity of forearm muscles both before and 3.5-5 mo after the lesion. 2. The lesion caused marked changes in movement kinematics and the patterns of activity in agonist, synergist, and antagonist muscles. 3. After the lesion, movements to all targets were performed more slowly. In addition, some movements were misdirected. For example, most movements to the target that required wrist flexion and radial deviation were made in two steps, instead of in a single smooth trajectory. 4. After the lesion, distinct bursts of muscle activity were no longer observed during step-tracking movements. In addition, suppression of antagonist activity at movement onset was abolished or reduced. The relative timing of agonist and synergist muscle activity was markedly altered. 5. We conclude that M1 contributes to the precise spatiotemporal patterning of muscle activity during step-tracking movements.