Seven well-trained male long-distance runners were studied during a 100-km road race. Hematologic parameters, plasma electrolytes, glucose, lactate, urea, and creatinine content in plasma and the activity of the enzymes γ-glutamyltransferase and creatine kinase were determined before and after the race. A slight increase in hematocrit was found after the race, although the red blood cell count and hemoglobin concentration remained unchanged. Further, a significant rise in the number of white blood cells, lymphocytes, and neutrophils was found after the race. Postrun concentrations of plasma sodium and potassium increased significantly from 142 ± 7 to 161 ± 7 mmol∙L−1, and from 4.22 ± 0.37 to 5.15 ± 0.46 mmol∙L−1(p < 0.05), respectively. Plasma concentrations of lactate (1.29 ± 0.31 vs. 3.57 ± 1.22 mmol∙L−1), urea (6.09 ± 1.0 vs. 8.35 ± 1.35 mmol∙L−1), creatinine (73.4 ± 3.5 vs. 117.6 ± 19.4 μmol∙L−1), plasma creatine kinase (91.1 ± 25.1 vs. 2843 ± 2341 IU∙L−1), and γ-glutamyltransferase (20.28 ± 1.88 vs. 24.14 ± 4.09 IU∙L−1) increased significantly (p < 0.05) after the run. It was concluded that during ultralong-distance races, acute renal dysfunction and muscle damage could contribute to the observed hypernatremia and hyperkalemia.Key words: ultralong-distance races, hematological parameters, plasma electrolytes, plasma creatinine, c