It is well known that chronic atrial fibrillation (CAF) and paroxysmal atrial fibrillation (PAF) are associated with a hypercoagulable state. However, pathological hemostatic changes during the paroxysmal supraventricular tachycardia (PSVT) have not yet been elucidated. To determine platelet activity in patients with PSVT, PAF and CAF, we examined the levels of beta-thromboglobulin (BTG) and platelet factor 4 (PF4) during tachyarrhythmia attacks. We measured the levels of BTG and PF4, as an index of platelet activation in 15 patients with PAF ( 9 men, mean age 45 +/- 11), and 14 patients with PSVT ( 8 men, mean age 40 +/- 10). Levels were compared to 22 age and sex-matched healthy controls in sinus rhythm and with 25 patients with CAF (16 men, mean age 51 +/- 12). Blood samples were taken during arrhythmia and 24 hours after conversion to sinus rhythm. Patients taking medications or have clinical conditions that may affect the BTG and PF4 levels were excluded.