Abstract Vitamin E is an important antioxidant for normal neuromuscular function in horses. Dietary deficiencies are common in northern climates with seasonal access to pastures. Certain breeds, such as Morgan Horses, with predisposition to equine metabolic diseases, may exhibit deficiencies in vitamin E because of management systems imposed which restrict access to pasture. Few studies have compared strategies for initial supplementation to increase serum alpha-tocopherol in horses. The objective of this study was to compare two dosing strategies of d-alpha-tocopherol acetate to increase serum alpha-tocopherol in marginal/deficient horses. Eight Morgan horses (two geldings, six mares; mean BW = 460 kg) with marginal/deficient serum alpha-tocopherol (0.47-1.12 µg/mL) were enrolled in a randomized block design study. Blocks were balanced for age, sex, and pre-study vitamin E level and horses were assigned to one of two treatments: 1) HIGH - 10,000 IU liquid d-alpha-tocopherol acetate top-dressed on grain for 4 wk followed by 10 d step-down to 5,000 IU of powder d-alpha-tocopherol acetate through 16 wk; or MEDIUM – 5,000 IU of powder d-alpha-tocopherol acetate for 16 wk. Horses were housed in box stalls with daily turnout without access to grass. Horses were fed hay at 2% of BW and 0.45 kg of concentrate daily with vitamin E feed activity of 45.5 and 1378 IU/kg, respectively. HIGH and MEDIUM treatments were fed once daily with the AM feeding. To ensure horses consumed the powder d-alpha-tocopherol acetate, cookies made from alfalfa powder, gelatin and d-alpha-tocopherol acetate powder were created. Serum alpha-tocopherol was assessed pre-feeding every two weeks from 0 to 16 weeks. Before the step-down to 5,000 IU dose, horses receiving the HIGH dosage had greater concentrations of serum alpha-tocopherol (P < 0.01; Figure 1). All horses receiving the HIGH treatment attained serum alpha-tocopherol concentrations greater than 2.0 µg/mL after 2 weeks, while only one-half of horses receiving the MEDIUM dosage reached serum alpha-tocopherol concentrations greater than 2.0 µg/mL by 4 weeks. To attain and maintain normal concentrations of serum alpha-tocopherol, deficient horses should follow the HIGH supplementing strategy of d-alpha-tocopherol acetate.