The purpose of this trial was to evaluate the possibility to use sampling of faeces, urine and plasma, and measurements of blood pressure as methods of discovering Hypomagnesemia in horses, as well as the application handiness of these methods. The most commonly used method of checking the magnesium status of a horse is to analyze samples of serum or plasma. The trial was of a switch-back design, in which the horses were used as their own controls. The animal material was five Swedish warm blooded trotters between six to ten years old, and weighing between 485-620 kg. The trial consisted of three periods that were two months each.