The effect of supplementation of limiting amino acids on rats fed a 10 or 20% wheat gluten diet containing 5% tyrosine has been investigated. Growth-limiting amino acids in a 20% wheat gluten diet were lysine and threonine. When rats were fed a 20% wheat gluten diet containing excess tyrosine, the addition of 1.0% lysine-HCl or 1.0% lysine-HCl and 0.4% threonine completely prevented the development of tyrosine toxicity, and was accompanied by a lowering of free tyrosine concentrations in plasma, liver and muscle. In rats receiving a 10% wheat gluten diet, lysine, threonine and methionine were limiting for growth. The addition of 1.0% lysine-HCl to the 10% wheat gluten plus 5% tyrosine diet failed to alleviate the tyrosine toxicity, but the supplement of 1.0% lysine-HCl and 0.4% threonine could partially alleviate the toxicity. The combined addition of 1.0% lysine-HCl, 0.4% threonine and 0.5% methionine was most effective. The free tyrosine concentrations in plasma, liver and muscle were lowered greatly by the supplementation of three limiting amino acids, but amounts of free tyrosine and phenolic metabolites excreted in the urine did not decrease with addition of these limiting amino acids.