Saliva is produced by large salivary glands (under and behind the jaw) that empty through ducts into the mouth. Normal horses produce a large quantity of saliva every hour, which they normally swallow. Saliva has very important functions. It wets feed material and begins to break it down. It also has an important buffering effect in the stomach, reducing acidity.

Drooling is an uncommon observation that can result from a variety of underlying causes including things like mouth injury or inflammation, a fungal toxin found in clover, inability to swallow, some infectious diseases, dental or oral conditions causing irritation, and as a response to contact with irritating or toxic materials. It can also be also seen in horses that are very ill from almost any life-threatening systemic (body-wide) condition. Those horses simply stop swallowing, allowing saliva to run back out of the mouth.

Sometimes drooling is seen in combination with nasal discharge. This usually indicates that a blockage is occurring in the throat or below. The best example of this is seen in horses that have "Choke"- esophageal obstruction. Drooling (along with nasal discharge) occasionally can occur when the small intestine or stomach is blocked and fluid accumulates in the stomach and finally backs up into the esophagus and then into the mouth and nasal passages.