1. Saliva flowed from the horse's parotid duct only during mastication.2. The surface-active local anaesthetic administered by mouth inhibited salivary secretion.3. Salivary secretion was stimulated by pilocarpine and inhibited by atropine.4. The volume and composition of saliva secreted in 24 hr from one parotid duct was determined.5. The concentration of sodium, potassium, calcium, chloride and bicarbonate depended upon the rate of flow. The highest concentrations of these electrolytes were observed during periods of high flow rates.6. Horse parotid saliva contained a high concentration of calcium.7. In the absence of a dietary supplement of sodium bicarbonate, the sodium concentration of the saliva fell after about 21 days. There was an associated increase in the potassium concentration. The addition of a sodium supplement restored the sodium concentration of the saliva within 24 hr.