In order to clarify the cause of white muscle disease (W.M.D.) in foals, tocopherol and selenium concentrations in serum and glutathione peroxidase activities in blood were measured. Examination was made on the samples from horses affected with W.M.D., the foal kept with them in the same stable, the foals kept in the stables without affected foals, and respective mares. The heavy-breed horses in Fukuoka prefecture and Tokachi district were also examined for comparison. Serum tocopherol levels of these foals were normal because after intake of colostrum. Mares of affected foals showed lower tocopherol levels than other examined mares (p<0.01). Serum selenium levels of all foals were below 65 ppb, showing deficient levels. The mares of affected foals had lower selenium levels than other mares (p<0.01). There was a good correlation between serum selenium concentration and blood glutathione peroxidase activity (r=0.81). Selenium levels in the liver of affected foals were lower than the foals which succumbed with other diseases. Diet supplied in the stables with affected foals showed low α-tocopherol and selenium contents. These findings suggest that W.M.D. in foals is attributed to nutritional muscular dystrophy caused by tocopherol and selenium deficiency of their mares in late gestation period.