An isotonic electrolyte solution with a composition similar to equine sweat was compared to an isotonic glucose-glycine-electrolyte solution for oral rehydration therapy in exercising horses. Ten horses were dehydrated by using frusemide and allocated randomly to receive 4 different oral solutions: isotonic sweat-like electrolyte solution, half-strength hypotonic electrolyte solution, isotonic glucose-glycine-electrolyte solution, and plain water. Solutions were given by nasogastric tube using the same volume as the bodyweight lost by each horse. Blood samples were collected before and throughout 6 h of the rehydration period. Results showed that all solutions recovered pre-frusemide values of packed cell volume (PCV) and total plasma protein (TP) in a similar fashion. No changes for Na+ values were observed during the rehydration period when the isotonic sweat-like solution was used. However, a significant hyponatraemia was induced throughout rehydration when the other 3 solutions were given, especially when hypotonic solution and water were used. Osmolality values did not change when both isotonic solutions were administered; but a significant hypotonicity was observed when hypotonic solution and water were given. When the isotonic sweat-like solution was used, plasma Cl-, K+ and creatinine values recovered to premedication values significantly faster than the other 3 solutions. In conclusion, the isotonic sweat-like electrolyte was the best solution because it restored rapidly the fluid and plasma electrolyte imbalances. In contrast, the isotonic glucose-glycine-electrolyte solution impaired the plasma electrolyte imbalances.