Magnesium (Mg) containing supplements and feeds typically claim, among many things, to have a calmative effect on horse behavior without any published evidence for such an effect. The aim of this study was to investigate if oral Mg supplementation above NRC recommendations increases the Mg status in the horse and has an observable effect on horse behavior. Six mature Standardbred geldings were used. Prior to Mg supplementation, all horses had their reaction speed recorded with and without mild sedation (0.04 mg/kg BW Acepromazine) using a random cross over design. Horses were then allocated a treatment using a Latin square design with each horse acting as its own control. All horses were fed a base (control) ration comprising low energy pellets (Cool Conditioner, CopRice, Leeton, Australia) plusclover/ryegrass hay fed at 2.0% BW (total mean Mg of ration: 11.2± 2.0 g). Three treatments were used in the trial: control ration, control plus 2.5 g added Mg (as magnesium aspartate) and control plus 10 g added Mg. Horses were fed each ration in 2 meals per day for a period of 7 d. All horses were kept in individual dry lots that allowed free exercise. On Day 6, a series of blood and urine samples were collected over 24 h to determine Mg status and excretion. On Day 7, each horse underwent a reaction speed test. This measured the time taken for a horse to cover 2 m in a custom-built chute after being startled. Repeated measures ANOVA of blood and urine concentrations at allocated sampling times showed an increase in total blood Mg concentrations and urinary Mg excretion between the control group and the Mg supplemented groups. There was a trend for Mg supplementation to decrease reaction speed (P ¼ 0.07) when the control was compared with both the 2.5 g and 10 g added Mggroups. There was a significant difference shown between the control and 10 g added Mg treatment (P ¼ 0.025) using a paired Students t-test. Mean reaction speed (±SD) was Control, 5.3 ± 2.2m/s; 2.5 g added Mg, 3.7 ± 1.8 m/s and 10 g added Mg, 3.1 ± 1.5m/s, with no effect of day. This was comparable to the pre-study testing where unsedated horses had a mean reaction speed of 5.3 ± 1.8 m/s compared with 3.5 ± 1.5 m/sec when sedated. This is the first time an objective measurement of behavioral change due to oral magnesium supplementation has been reported in the horse.