Twenty-three Standardbred mares (4 to 8 years old) were used to determine the effects of season on mineral metabolism. Diet was formulated to supply recommended protein and energy requirement.(1) No additional mineral was supplied in the diet. Blood samples were collected from the horses at the beginning and the end of January, April, July, and November. In addition, hair samples from 8 different parts of the body were collected on January and July. Mineral concentrations of feeds were higher in summer months than those in other seasons and lower in autumn than those in other seasons. Serum copper and zinc levels were the highest in the summer months. Serum iron levels were the highest during the spring. Except for copper, mineral contents of the hair were significantly lower in winter than those determined in summer months.