The relation of urea nitrogen (UN) to nonprotein nitrogen (NPN) was studied in 183 patients with unselected fatal diseases (598 pairs of observations), with control by autopsy. When the value of NPN was over 100 mg./100 ml. blood, 85% of the cases were renal patients. The variation of UN with NPN (azotemic ratio) was linear over the entire range of observations, the equation of this line and other statistics being graphically reproduced, with a confidence band, including 97% of the observations. The avg. variation of the azotemic ratio was computed; it depends on the NPN and has no diagnostic value in renal disease. To ascertain whether there is N retention, it is sufficient to determine either UN or NPN; the increment of UN is, on the avg., 86% of the increment of NPN at all levels of N retention. Similarly, an increment of 100 mg. of UN corresponds, on the avg., to an increment of 16 mg. of the non-urea components of the NPN. In N retention the UN is thus in equilibrium with the other component''s of the NPN.