BACKGROUND AND OBJECTIVES: Regular aerobic exercise plays a role in the prevention of cardiovascular diseases, but the risk of primary cardiac arrest transiently increases during vigorous exercise, particularly in sedentary men. There has been a controversy regarding the effect of exercise on platelet functional behavior, which is related to the pathogenesis of coronary ischemia. We evaluated the cytoplasmic free calcium concentration of platelets in healthy men after treadmill exercise. MATERIALS AND METHOD: Five physically active (group I: age, 24.2+/-2.3 years) and five sedentary men (group II: age, 22.4+/-1.4 years) were included in this study. Platelet calcium was measured with fluorescent dye, quin2 before and after treadmill exercise. RESULTS: Platelet calcium levels were increased from 86.8+/-11.8 nM to 128.8+/-15.0 nM in group I (p<0.05) and from 102.6+/-14.4 nM to 162.4+/-26.5 nM in group II (p<0.05) immediately after exercise. Thereafter, platelet calcium levels were decreased in group I, but continuously increased in group II. At fifteen minutes after exercise, platelet calcium concentrations were significantly higher than baseline values in group II (205.8+/-53.9 nM vs 102.6+/-14.4 nM: p<0.05), but not in group I (115.2+/-15.7 nM vs 86.8+/-11.8 nM). CONCLUSION: Cytoplasmic free calcium concentration in platelets were continuosly increased after treadmill exercise in sedentary men but not in physically active men.