1. In a series of non-nephritic individuals the total non-protein nitrogen of the blood, determined by Folin's method, was found to lie between 15 and 43 milligrams per 100 cubic centimeters. From 50 to 60 per cent. of this was in the ammonia-urea fraction.
2. In cardiovascular disease with renal congestion, but without other renal lesion, there was no evidence of increase of non-protein nitrogen in the blood, nor of alteration of the ammonia-urea percentage.
3. In chronic nephritis with marked albuminuria and edema there was very little, if any, increase or alteration.
4. In chronic nephritis with hypertension the non-protein nitrogen was definitely increased, ranging from 40 to 180 milligrams per 100 cubic centimeters of blood. The percentage of the ammonia-urea fraction was usually higher than in non-nephritic cases.
5. Cases showing high non-protein nitrogen values were subject to rapid fluctuations in these values in the course of a few days. As a rule, clinical improvement was associated with a fall of the non-protein nitrogen figures to nearer the normal range.
6. Uremia was almost always accompanied by an increase of non-protein nitrogen in the blood, but no constant relation could be established between the degree of increase and the tendency to uremia.
7. Our cases have not yet been followed for a long enough period to admit of conclusions as to the possible relation between the degree of non-protein nitrogen retention and ultimate prognosis.
8. We believe this method to be a valuable aid in the clinical study of nephritis and that it can be readily carried out in any well equipped clinical laboratory.