A large body of evidence has shown that the observation of actions done by others evokes a subliminal activation in motor pathways of observers (motor resonance), mediated by the mirror neuron system. In all published studies subjects are always instructed to observe an action and thus their attention is explicitly focused on it. What is the role of attention in the development of the motor resonant response? Is it necessary that the observer's attention be explicitly focused on the action? Or is it sufficient that observers view the action, though their attention is diverged by some other stimulus? We explore these questions by asking 14 subjects to observe a 1Hz flexion-extension hand movement (explicit observation), and 14 different subjects to count how many times a small light, on the dorsal surface of the experimenter's hand, is turned on, while the experimenter performs the same hand movement (implicit observation). Motor resonance is measured as the modulation of the H-reflex in the Flexor Carpi Radialis muscle (FCR) of observers. The number of times the light is turned on varies randomly in each trial (100); the task is fairly easy, but requires constant attention. Explicit observation evokes a significant (R2=0.42; p<0.0001) sinusoidal modulation in the observer's FCR H-reflex, while during implicit observation the reflex modulation is lost (R2 = 0). We conclude that the role of attention is essential in the development of the motor resonant response.