Platelets are known to become activated in vivo by different stressful stimuli such as surgery and dynamic exercise. Mental stress has been shown to increase platelet aggregability. Platelet activation is thought to be of major importance in atherogenesis and cardiac fatalities. In order to clarify further stress-induced platelet activation with special reference to the period after the stress, we studied eight young, healthy volunteers during and for 1 h after a mental stress test (Stroops Colour Word Conflict Test). Using highly standardized techniques, we have measured platelet aggregability ex vivo and the platelet release products beta-thromboglobulin, platelet factor 4 and serotonin in plasma. As markers of the stress response we measured cyclic-AMP in plasma, heart rate, cardiac output and blood pressure. The stress test induced a significant cardiovascular response with increases in heart rate, blood pressure, and cardiac output and as a measure of adrenergic activity an increase in cyclic AMP in plasma during the test. Platelet aggregability was unaffected during the test but decreased following the stress. During the first hour following the test and release products beta-thromboglobulin and serotonin increased significantly in plasma. We conclude that platelets are activated during mental stress and that this activation involves a post-stress release of vasoactive compounds from platelets.