A basic diet is described containing (with arrowroot starch) not more than 0.2 gm N for the average individual, which has been used with but slight changes over a period of nearly 20 months. This diet constituted the “no-protein” regime in 12 different periods for determination of endogenous nitrogen excretion. It was found that the following factors influence the time within which a dependable endogenous urinary nitrogen could be obtained within 3 or 4 days, on a no-protein diet: (a) the level of protein in the pre-experimental diets; (b) the position of the no-protein period in the series of periods; (c) the nature of the protein, called here “supporting protein,” immediately preceding the no-protein, and its level of intake; and (d) conditions antecedent to the supporting protein which could affect the accrued nitrogen deficit to the beginning of the no-protein period. Dependable and stable endogenous nitrogen levels can be reached on the fourth day of no-protein, even if the nature of the supporting protein and its level of intake provide a fairly stable deposit of reserve protein, but the total antecedent conditions bring the average subject, starting fresh, to a state of accrued nitrogen deficiency equal to about 40 gm. Stable endogenous nitrogen excretion can be reached on the third day of no-protein provided the total antecedent conditions bring the average subject to a state of accrued deficiency equal to about 60 gm nitrogen. Some evidence has been developed that subjects acquire a resistance to the depleting effect of a no-protein diet after having passed through some months of low protein diets, including five or six no-protein periods. Blood examinations for hemoglobin and serum protein prove that members of a diet squad passing through at least 6 months subsistence on alternating high and low value proteins at a general level of not to exceed 5% of the calories can withstand at least six no-protein periods of 3 days each within that period without apparent injury to the liver, so far as its protein-forming functions are concerned. Regression equations are given of endogenous N on body weight in 73 determinations of the urinary excretion on 28 men and in one each on seven women for the third-day no-protein diet as well as in 22 determinations on 14 men for the fourthday no-protein. The number of observations on women is too small to derive an entirely satisfactory equation. In duplicate observations of basal metabolism on five men at the termination of the fourth-day no-protein in two periods 1 month apart, the average coefficient, milligrams endogenous urinary nitrogen to kg-cal. of the 24-hour basal heat, was found to be 1.48 and 1.34, respectively — values which agree much more closely with those of Deuel, Sandiford, Sandiford and Boothby and with two on women reported recently by Bricker, Mitchell and Kinsman ('45) than with those of Terroine or Smuts.