Based on the Implicit-Affect-Primes-Effort model (Gendolla, 2012, 2015), we tested whether warning individuals about the occurrence of affect primes during a cognitive task moderates the primes' effect on effort-related cardiac response. Participants worked on a challenging mental arithmetic task with integrated masked affect primes—very briefly flashed pictures of facial sadness vs. happiness expressions. Additionally, half of the participants were warned about the primes' appearance and their possible effect on experienced task demand; the other half of the participants was not informed about the primes. Reactivity of cardiac pre-ejection period (PEP) was stronger in the happiness-prime than in the sadness-prime condition, but only when the participants were not warned about the primes' presence. This effect was further moderated by gender and only significant among men. Heart rate (HR) responses showed a largely corresponding effect. The results suggest that prime-warning is a boundary condition of implicit affects' effect on effort mobilization—and that this effect applies especially to men.