In order to determine if the stress of a standard exercise test will affect the propensity for platelets to aggregate, platelet aggregometer studies were performed before and after a double two-step Master's test in 19 patients with angina and in 19 normal controls. The response of platelet-rich plasma to 0.1, 1 and 10 muM of ADP and 1 and 10 muM of epinephrine were tested in a Chrono-Log aggregometer. Prior to exercise there was no significant difference between the number of patients and controls responding with irreversible aggregation to any of the five tests. Immediately following exercise, no significant differences in the number of irreversible aggregation responses were seen in the control group. In the group with coronary artery disease, the incidence of irreversible responses was significantly higher after exercise on test with 1 muM ADP (p less than 0.05), 1 muM epinephrine (p=0.02), and 10 muM epinephrine (p less than 0.05). It is concluded that exercise increases significantly the tendency of platelets to aggregate among patients with coronary artery disease. It is possible that this effect may play a role in myocardial infarction induced during stress.