The review focuses on fluid intake, weight changes and blood and urine parameters in ultra-endurance cycling and running performances. In ultra-marathoners, the prevalence of exercise-associated hyponatremia (EAH) varied between 0 and 44 %. EAH is the occurrence of hyponatremia during or up to 24 hours after prolonged physical activity and is defined by a serum or plasma sodium concentration ([Na⁺]) below the normal reference range of the laboratory performing the test. Early signs and symptomps of EAH can include bloating, nausea, vomiting and headache; however, many of these signs can develop as a result of celebral edema, seizures, respiratory distress, coma and death. Risk factors for fluid overload and subsequent EAH are a slow exercise pace, high frequency of fluid intake, low body mass, long duration of exercise, altered kidney excretory function and unusually hot or extremely cold temperatures during competition. We compared partial results of our research in connection with findings in present studies and confirmed theories about differences in physiological responses in athletes between performance with breaks (a running multi-stage marathon) and non-stop performance without defined breaks (a mountain biking 24-hour race). As no large weight changes were noted in cyclists, indicating that significant dehydration and over-hydration were unlikely to have occurred. On the contrary significant changes in body mass in runners reflect that fluid substitution was not effective enough to prevent exercise induced dehydration. However due to the significantly decreased hematocrit, there was the possibility of a decrease of skeletal muscle mass.