It is known that there may be a variation of more than 100 per cent. in the blood urea concentration of any group of normal subjects. It is not known why this considerable degree of variation should exist. From the available data it is possible to suppose that each person has his own characteristic blood urea concentration, which varies only within narrow limits, although the variation between different subjects is so large. Or it may be that the blood urea concentration in each person varies widely in accordance with one or other of the many constantly changing conditions of the body.
One cause of variation in the concentration of some constituents of the blood of different persons is a variable food intake. And before any study of the effect of physiologic variables on the blood urea concentration can be carried out or any attempt made to draw