In drinking, the action of the mouth is that of a suction pump, in which the tension of the air in the mouth is reduced by the tongue being drawn back while the lips are kept closed. If this is done while the horse's muzzle is immersed in the fluid and the lips are separated without any air being admitted, the fluid will rush into the mouth and will rise to a height sufficient to render the pressure of the fluid on the air in the mouth equal to that of the atmosphere, and also sufficient for the swallowing muscles to force the fluid into the gullet. In this process, the action of inhaling (drawing in the breath) appears to have little or no effect; for we find that a horse which has an open tracheotomy tube in his windpipe will drink as easily as a sound horse, other conditions being equal. In drinking, a horse naturally keeps his nostrils above the surface of the fluid, as he has to breathe through them; but while his nostrils are un-immersed, the corners of his mouth will also be above the surface of the fluid. Hence, if the lips are separated at their corners, as for instance by the wearing of a curb bit, and especially if the reins are drawn more or less tightly, the animal will be greatly impeded in drinking, even if he is not altogether prevented from swallowing the fluid. A snaffle is less liable to cause interference in this manner than a curb; because it is as a rule put lower down in the mouth, and if jointed, it hangs down by the bending of its joint, and does not draw the lips apart so much as the mouthpiece of a curb would do.

According to Colin, a horse when drinking, takes from .26 pint to .44 pint at each "go-down" (swallow), and drinks at the rate of from 65 to 90 go-downs a minute. The quantity of an average go-down would therefore be about 1/3 pint. When thirsty, a horse would drink a gallon of water in about 20 go-downs.