To determine the involvement of activated platelets in the frequent thrombosis after hot hot-spring bathing, we examined effects of a hyperthermal stress on platelets in vivo. Plasma levels of beta-thromboglobulin (beta-TG) began to rise at 5 minutes and elevated significantly 10 minutes after the start of traditional 47 degrees C hot-spring bathing. In contrast, there was no significant factor 4 change in plasma beta-TG level through a control 10-minute bath in the same water at 42 degrees C. A beta-TG to platelet factor 4 ratio, a value suggested to be useful for evaluating clinical studies for in vitro versus in vivo granule release, was sufficiently high in each sampling, indicating that the experiment was performed without being influenced by artificial platelet activation. These data show that only a few minutes' hyperthermal stress can induce granule release from platelets in vivo.