Work on physiological and other behavioral correlates of motives often assumes that motives exert a direct effect on behavior once activated. Motivational intensity theory, however, suggests that this does not always apply. In the context of task engagement, motive strength should exert a direct effect on myocardial beta-adrenergic activity if task difficulty is unclear, but not if task difficulty is known. The presented study tested this prediction for the impact of the explicit achievement motive on myocardial beta-adrenergic activity — assessed as pre-ejection period (PEP) reactivity during task performance. Seventy-eight participants performed one of two versions of a mental arithmetic task. After having completed the achievement motive scale of the Personality Research Form, participants were either informed about the difficulty of the task or not before working on it. Participants' PEP reactivity during task performance provided evidence for the predicted moderating impact of clarity of task difficulty: PEP reactivity increased with increasing achievement motive strength if task difficulty was unclear, but not if it was clear. These findings demonstrate that the explicit achievement motive impact on myocardial beta-adrenergic activity is moderated by clarity of task difficulty and suggest that motive strength does not always translate into direct effects on physiology and behavior.