1. Plasma amino acids and six proteins have been measured in patients with chronic renal failure receiving low protein diets before and after oral supplementation with essential amino acids. 2. All the patients on low protein diets had a lower percentage of essential amino acids in their plasma than normal subjects but after supplementation, plasma levels increased significantly with minimal increase in non-essential amino acids or urea nitrogen. 3. Mean levels of plasma transferrin, complement C3 and globulin Gc were lower and plasma prealbumin higher in patients than in normal subjects. Plasma complement C4 and albumin were not different from normal. 4. Seven out of nine patients who tolerated the supplementation showed a significant increase in plasma transferrin, prealbumin and complement C3 but not in complement C4, globulin Gc or albumin. 5. Correlations between the percentage of essential amino acids and each of plasma transferrin, prealbumin and complement C3 and also between several of the plasma proteins further substantiate their value in the assessment of dietary intake in chronic renal failure. 6. The value of amino acid supplementation on low protein diets in chronic renal failure is discussed in relation to these observations.