1. l-Valine, l-leucine and l-isoleucine containing 15N were prepared, purified, and fed to marasmic and recovered infants. The excretion of total 15N and [15N]urea in the urine was followed for 72 h.
2. 15N excreted from isoleucine was about 90%, from leucine 70%, and from valine 50% in the recovered child.
3. 15N excreted was lower for leucine and isoleucine in marasmus than in recovery. The clinical state made little difference to 15N excretion from valine.
5. The decrease in excretion was almost entirely due to lowered urea output.
6. The results are discussed in relation to branched-chain amino acid metabolism and dietary requirements for these acids.