This study examined the hypothesis that a diet containing excess leucine may promote protein deposition in the body of rainbow trout. Diets were formulated with wheat germ meal and crystalline amino acids as major nitrogen sources. In Experiment 1, diets containing 1.1, 1.5, 2.2, 2.7, 3.5, 4.5, 6.0 and 6.5% leucine in wheat germ meal-crystalline amino acid diets were fed to fingerling rainbow trout. Diets containing up to 6.5% leucine did not inhibit weight gain or food intake. Body protein concentration tended to decrease as dietary leucine increased. In Experiment 2, fish were fed similar diets containing 3.3, 6.2, 9.2 and 13.4% leucine. After 10-11 wk of feeding, gross lesions including scoliosis, deformed opercula, scale deformities, scale loss, spongiosis of epidermal cells and scale regeneration were observed in 20% of the fish fed diets containing 13.4% leucine. High dietary leucine did not depress plasma valine or isoleucine concentrations. Therefore, the gross lesions could be attributed to a toxic effect of excess dietary leucine. Polyamine concentrations, which were used as a metabolic indicator for growth, were not significantly different in the tissues of fish receiving different treatments, thus supporting the hypothesis that increasing dietary leucine did not increase body protein deposition.