Nutritional adequacy of growing Thoroughbred horses raised in an alternate feeding system - grazing during late spring through late fall and stable feeding for the rest of seasons - was assessed by determining vitamin E and trace mineral levels in the serum and blood chemistry related to nutrition and health. During the stable feeding in winter and early spring, 50 growing female horses were fed concentrates (1.4% of their body weight), grass hay (0.62%) and alfalfa hay (0.37%). For the grazing period, the same horses were fed supplementary concentrates (1.1%) during late spring through early summer, and concentrates (1.1%) and alfalfa hay (0.5%) during late summer through late fall. Blood samples were collected before grazing in early spring, and during grazing in early summer through late fall. Serum vitamin E, BUN, GTP, total bilirubin and direct bilirubin levels were increased (P<0.01) by grazing compared to those measured before the initiation of grazing. Horses had lower (P<0.01) serum Fe contents in early summer than in late fall or in time of stable feeding. Stable feeding increased (P<0.01) serum Cu content compared to grazing in both early summer and late fall. In late fall, serum Zn level increased (P<0.01) compared to that found in the other seasons. Blood glucose and creatinine levels decreased (P<0.01) after grazing. Results indicate that supplementations of some minerals and vitamin E are not always necessary in diets for growing horses and should be done after careful evaluation of diets with regard to concentrations and biological availability of minerals.