Can evolving motor commands be interrupted by changes in sensory signals that triggered them? We investigated this problem by observing the changes in single cell activity in the motor cortex of monkeys, changes that preceded movement of the hand toward a visual target. We found that this activity was interrupted following a shift of the target during the reaction or movement time and replaced by the pattern activity related to the movement towards the new target. This suggests that motor cortical commands subserving aimed arm movements are processes that can be interrupted in the course of their formation and/or execution by changes in afferent controlling inputs.