Blood platelets have been found to play a major role in the pathogenesis of coronary artery disease. This study examined the effects of active and passive stressors on platelet function and assessed the possible effects of perceived control on platelet reactivity to stress. Heart rate, blood pressure, catecholamine levels, and platelet factor 4 (PF 4) were assessed in 55 healthy males during resting baseline (25 min), 7-min stressor/task periods, and recovery (60 min). Results showed significant increases in PF 4 during the stressor/task periods for both active and passive stressor groups. Psychological stress involving both active and passive participation had a direct effect on platelet activation.