Rules and instructions can exert a powerful influence on behavior, even when they have never been followed before. Existing evidence suggests that the processing of a rule results in a state of mental preparedness that enables efficient and automatic response activation. We present results supporting the idea that that this state is partly achieved by spontaneous mental simulation of responding to the instructed stimulus-response rule. Using electromyography, I compared response-specific muscular activity in the right and left forearm during the presentation of instructions mapping a right or a left key press. As predicted by the mental simulation hypothesis, activity was higher during congruent than incongruent instructions in both forearms. Mental simulation, therefore, seems to be a promising mechanism to explain how rules become incorporated in the human cognitive system.