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1. Rings of rat jejunum incubated in vitro accumulate a mixture of amino acids at a rate of about 3 µmoles/cm. hr. The rate of incorporation of the accumulated amino acid into the tissue protein corresponds to a rate of synthesis of 50% of the protein of the whole wall in five days.

2. Replacement of the Na⁺ in the NaCl of the incubation medium by choline or by Li⁺ did not prevent amino acid accumulation by the tissue. However, replacement of the Na⁺ by K⁺ prevented the accumulation.

3. The accumulation of amino acids by rat jejunum in vitro proceeded at normal rates not only in the presence in the incubation medium of oxygen tensions below 10 Torr but also in the presence of 2,4-dinitrophenol. Reasons are given for supposing that the findings are compatible with the view that the energy upon which depend the processes of amino acid accumulation by the tissue could be derived from the movement of ions across cellular boundaries.

4. The amino acid incorporation into the tissue proteins was reduced to one tenth of the control rate in the presence of 2,4-dinitrophenol or by hypoxia so that the processes of incorporation depend upon energy derived from oxidative metabolism. In the presence of oligomycin the tissue respiration was depressed but the amino acid incorporation into the tissue protein was not inhibited. The view that the amino acid incorporation may be able to function with energy intermediates other than ATP is discussed.