To evaluate the effects of mental stress on blood platelet activity, platelet secretion and aggregation were measured in 40 healthy young men, assigned in a 3:1 ratio to an experimental and control condition. After a baseline period, experimental subjects participated in a 21-minute, frustrating computer task (the Stroop test), while control subjects remained seated quietly for the same duration. Blood was drawn from all subjects immediately before and after the task period for assessment of platelet activity (secretion of ATP and aggregation in response to 5 and 20 microM ADP). Heart rate and blood pressure were also assessed at baseline and throughout the task period. Results indicated that measures of platelet secretion, heart rate, and blood pressure rose significantly from baseline to posttask assessments in subjects exposed to the experimental stressor (p < .05), but not among controls. These findings are consistent with the hypothesis that stress may potentiate coronary disease pathogenesis, in part, via activation of blood platelets and their associated effects on coronary artery occlusion and/or constriction.