Now that marathon racing is growing in popularity, many thousands of enthusiastic athletes are participating in various ultramarathons all over the world each year. However, it remains controversial whether such a sport contributes to the promotion of health. The occurrence of transient cardiac dysfunction and irreversible myocardial injury has been reported in association with such exercise in healthy individuals. Brain natriuretic peptide (BNP) is a cardiac hormone, as is atrial natriuretic peptide (ANP), and its measurement has been widely used for clinical evaluation of cardiac dysfunction. However, little is known about the response of plasma BNP to prolonged strenuous exercise. We hypothesized that confirmation of minimal cardiac dysfunction or myocardial injury may be made by measurements of plasma BNP. Levels of plasma ANP, BNP, catecholamines, blood lactate, and serum cardiac troponin T (cTnT) were determined before and after a 100-km ultramarathon in 10 healthy men to examine the effects of the exercise on levels of ANP and BNP and correlations between the natriuretic peptides and cTnT as a marker for myocardial damage. Whereas all variables significantly increased after the race, increased levels of ANP and BNP were most strongly correlated with increases in cTnT levels. The cTnT level after the race was greater than the upper reference limit in 9 of 10 men. Such exercise significantly increased ANP and BNP levels in healthy men, and the increases could be partially attributed to myocardial damage during the race.