By borrowing from the biopsychosocial model of challenge and threat, we may understand when health communications adequately motivate behavior change or when they are overly distressing and inhibit behavior change. The present studies were guided by the biobehavioral model of persuasion, which predicts that different health appeals should evoke different appraisals, as well as psychological and physiological responses that motivate the adoption or rejection of health behaviors. Challenging messages should be associated with approach motivational tendencies, whereas threatening messages should be linked to avoidance. Findings from 2 experiments support the biobehavioral model of persuasion and offer mechanisms that confer message effectiveness. By understanding the psychological and physiological processes that drive message effectiveness, message designers can create more effective health appeals.