We tested the hypothesis that during infusion of amino acids without dextrose, there is less insulin stimulation, which, in turn, permits lipolysis. The results suggest that dextrose infusion stimulates insulin and inhibits lipolysis. During administration of crystalline amino acids without dextrose, nitrogen balance improved substantially from control values obtained during dextrose infusion, while the level of serum-free fatty acids and ketone bodies rose and that of serum immunoreactive insulin fell. Infusion of amino acids at 1.7 gm/kg appeared slightly more efficient than infusion at 1.0 gm/kg and did not stimulate insulin or inhibit lipolysis. Protein sparing may be useful in certain specific clinical situations. However, the results must be interpreted cautiously, primarily because it is difficult to establish the relationship between improvement in nitrogen balance and the derived clinical benefit. Absolute proof of the efficacy of this technique awaits further studies using indexes that more accurately measure protein synthesis.