According to earlier observations no further augmentation of urea clearance should be possible above a urine flow of 2 ml/min (so-called augmentation limit). In normal people and in patients with renal insufficiency no augmentation limit of urea and creatinine clearance could be ascertained: increasing urine flow was correlated with augmentation of urea and creatinine clearance. Calculating the protein equivalent the augmentation of urea excretion by increased diuresis seems to be of clinical importance only in the treatment of renal insufficiency with small basic diuresis.