Horses sweat to stay cool and return their body temperatures and heart rates to normal when it's hot outside or after exercise. A horse can sweat up to 4 litres per hour while it is working out. Horse sweat, however, differs from human sweat in that it has higher levels of water and electrolytes. To prevent your horse from collapsing or foaming at the mouth, it is essential that these electrolytes be supplied. Make sure your horse drinks frequently to prevent electrolyte loss and excessive sweating (around 10 to 20 gallons a day). You can buy electrolytes and add them to your horse's drink if hydration is insufficient to keep them well hydrated.