BACKGROUND: Postexercise proteinuria, hematuria and changes in serum electrolyte balance as well as increased levels of plasma indicators for muscle leakage are believed to be transient and of benign character. METHODS: A group of 51 healthy athletes took part in a 100 km race over 14.25 hours. All of them had to reach the finish together. Urine and blood samples were collected before (a) and immediately after running (b) as well as 6 hours after the race (c). RESULTS: The serum concentrations of potassium (4.8 +/- 0.5 (a) vs. 4.0 +/- 0.3 (c) mmol/l), protein (73.1 +/- 5.2 (a) vs. 71.1 +/- 3.9 (c) g/l) and albumin (44.0 +/- 2.85 (a) vs. 42.9 +/- 2.8 (c) g/l) decreased significantly (p < 0.0001, p < 0.05, p < 0.05, respectively) but remained within physiological ranges. The serum sodium concentration decreased immediately after the race (136.9 +/- 4.5 (a) vs. 131.1 +/- 2.4 (b) micromol/l, p < 0.0001). The fractional sodium excretion decreased 6 hours, but not immediately after the race (0.78 +/- 0.59 (a) vs. 0.48 +/- 0.82 (c), p < 0.05). Myoglobin (31.8 +/- 6.9 (a), 291.5 +/- 197.2 (b) and 182.2 +/- 135.3 (c) microg/l, p < 0.0001) and creatine kinase (1.13 +/- 0.45 (a), 10.76 +/- 6.9 (b) and 9.46 +/- 15.5 (c) pmol/l, p < 0.0001) increased dramatically. Troponin I was also significantly increased at finish (0.0186 +/- 0.0121 (a) vs. 0.0213 +/- 0.0165 (b) ng/ml, p < 0.05) and positively correlated with myoglobin and creatine kinase, but remained far below the pathologic range. Serum creatinine and urea remained almost unchanged. Glucosuria and hematuria occurred 6 hours after the run in 9.1% and 6.8%, respectively. The erythrocytes examined by phase-contrast microscopy were not damaged in terms of dysmorphic cells. Glomerular-type proteinuria was found in 11.4% of the participants 6 hours after the race. CONCLUSIONS: We conclude that long lasting, mild exertion is harmless for renal function, electrolyte balance and skeletal muscle as well as myocardial metabolism in healthy persons.