In man the opinion that urea clearance is somehow dependent on urine flow is well-established. But this finding is only valid for subjects with normal renal function. There exists a statistically highly significant hyperbolic correlation between the clearance ratio of urea and creatinine and the concentration ratio of creatinine between urine and plasma (U/P(Kr)) as an approximate measure of the renal concentration operation. In renal diseased patients, however, with moderate reduction of the glomerular filtration rate down to slight renal insufficiency this relationship could neither be proved in preceeding investigations nor in this paper.