Weanling rats were offered a choice of two diets, varying only in lysine content. The preference test was performed between a 20% gluten diet supplemented with 0.5% lysine•HCl and a lysine-deficient amino acid mixture, 20% gluten or 20% gluten plus 0.2% lysine•HCl diet, and was done between a 20% gluten diet and a 20% gluten diet containing 3, 5 or 7% lysine•HCl. Weight gain and food intake were not significantly different among all the self-selecting groups, and these values were almost the same as those in rats fed only the 20% gluten supplemented 0.5% lysine•HCl diet which obtained maximal growth. The lysine intake of the combination groups ranged from 67 to 240mg per day (0.62-2.35% of consumed food). It was demonstrated that rats exhibit a definite ability to regulate lysine intake, and they select sufficient lysine to meet their requirements for this amino acid. The self-selection technique may be useful as a method to determine the requirements for amino acids.