1. Plasma sulphur-containing amino acids were measured in 19 patients with renal failure on chronic haemodialysis and in 22 normal subjects, to determine the rate of accumulation of these amino acids in chronic azotaemia.
2. Cysteine-homocysteine mixed disulphide was significantly increased in patients before dialysis and homocystine was detected in low concentration in 10 patients. Cystine and taurine were also increased. Changes in other neutral and acidic amino acids were similar to those reported in chronic renal insufficiency.
3. In 3–4 h of dialysis serum creatinine was decreased by a mean of 55%, cysteine-homocysteine by 41%and cystine by 58.5%(P<0.001 for each). Methionine concentrations were normal throughout.
4. We conclude that sulphur-containing amino acids, except methionine, accumulate in chronic renal failure as rapidly as creatinine.
