Immature male albino rats (70-80 g), eating a diet furnishing all nutrients for optimal growth and containing 84.6 mumoles of leucine/g, gained weight at an average rate of 5.9 g/day. Removal of leucine caused an average daily weight loss of 1.3 g. Addition of leucine to the diet in graduated amounts corrected this weight loss, the improvement in body weight (BW) being proportional to the dietary content of leucine. Addition of alpha-ketoisocaproic acid in place of leucine to the leucine-free diet also prevented loss of BW. Percentage efficiency of alpha-ketoisocaproic acid as a dietary substitute for leucine was calculated as: (see article). Efficiency of such substitution varied from 27% for a diet containing 84.6 mumoles of alpha-ketoisocaproic acid/g to 20% for a diet containing 169.2 alpha-ketoisocaproic acid/g.