Based on the Implicit-Affect-Primes-Effort model (Gendolla, 2012), we tested whether implicitly processed affect primes' effect on cardiovascular responses is limited to settings that call for effort and in which implicit affect can inform about subjective task demand. Participants were presented with letter series and briefly flashed sadness vs. happiness primes. Half of the participants were asked to memorize the presented vowels (achievement context), while the other half merely watched the series (watching context). Responses of cardiac pre-ejection period, heart rate, systolic, and diastolic blood pressure supported the predictions. As expected, in the challenging achievement-context condition, happiness primes led to stronger cardiovascular reactivity than sadness-primes. By contrast, reactivity was modest in both affect prime conditions when the participants merely watched the stimuli. That is, affect primes' impact on cardiovascular responses was limited to a setting that directly called for effort mobilization.