Background: The recovery of acid-base and hydro electrolytic balance at medical discharge after Raid races and its association with comfort index (CI) is unknown. Aim: To determine hydroelectrolytic and acid–base variations before and at the time of clinical discharge in horses that completed 90 km Raid competitions. Methods: Hematocrit, total protein, sodium, potassium, chloride, calcium, and serum pH were determined in samples before and after the race and after hydration in 276 horses and were analyzed using a mixed procedure, including the time of the race, the CI and its interaction. Results: The hematocrit and total protein values after hydration showed a correction to normal values, however, while for total protein no differences were observed between this sample and before the race, the hematocrit at medical discharge was still higher than the sample before the race. The pH values after the race were higher than before the race and after hydration. Sodium concentrations before and after the race were higher than after hydration. Chloride and calcium concentrations before the race were higher than after the race and hydration. Potassium concentration was affected by the interaction between CI and sampling, as only in worse climate conditions, potassium concentrations after the race and hydration were lower than before the race. Conclusion: Most hydroelectrolytic and acid-base variables at the medical discharge did not reach the values before the race, but they were within the normal values. The CI was a non-explanatory source of variability for most variables in this study.