This investigation of the relationships between the urea and creatinine contents of blood was made because of a number of apparent anomalies which have been encountered and which could not be explained on the basis of technical errors. The results of this study are of clinical interest since they lead to a new interpretation of these tests. A glance at a large series of determinations of the urea nitrogen and creatinine contents of specimens of blood reveals some parallelism between the concentrations of these two nitrogenous substances, though they are not strictly proportionate. For example, in table 1