Integrating the achievement motive literature and motivational intensity theory, we expected the implicit achievement motive (nAch) to directly determine effort mobilization when task difficulty is unclear. However, nAch should interact with task difficulty in determining effort mobilization when task difficulty is clear. Par- ticipants worked on an easy versus difficult memory task (Study 1) or a clear versus unclear arithmetic task (Study 2). We used the Picture-Story-Exercise to assess nAch and pre-ejection period (PEP) to operationalize effort. As predicted, PEP reactivity was strong in the difficult-high-nAch condition and in the unclear-high-nAch condition but low in the other three conditions. Supporting motivational intensity theory, our results showed that nAch requires difficult or unclear task conditions to exert a noticeable impact on effort.