One of the obvious uses which may be made of a tracer in biological studies is the determination of rates. Zilversmitt and associates (1) and more recently London (2) have called attention to this use of a tracer. Only when the isotope can be added without effectively changing the amount or concentration of material in a steady state system can such an application be made. Not until recently has radio iron been available which had sufficiently high specific activity to tag plasma iron without changing its concentration. The amount of iron leaving and entering the plasma would be significant in determining an abnormal turnover rate in some other system of the body containing iron, for example, the red cells. The present theory of iron metabolism conceives of plasma as a pool into which iron is returned before being resynthesized into the complex organic substances, hemoglobin, myoglobin, cytochromes, peroxidases, and ferritin which are so important to body function. Since the approximate normal rate of turnover of red cell iron is known, and since the major portion of this element in the body resides in the red cells, it would be expected that abnormalities in this rate would be directly reflected in plasma iron turnover rates. This paper concerns the plasma iron and red cell iron turnover data on 75 human subjects who were given amounts of iron which did not alter the steady state systems. It is shown that such turnover rates do, indeed, agree with the clinical and laboratory data concerning normal red cell life, abnormal rates of destruction and abnormal rates of formation of red cells. It is believed that the data ascertained from the type of study described here are of value

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The over-all metabolic activity of adipose tissue in man and other mammals has not been defined as carefully as that of other major organs and tissues. Despite recent reports (1, 2) showing that adipose tissue partakes in many biochemical reactions common to other tissues, widespread opinion holds that fat depots serve little purpose other than storage.A primary index of metabolic activity in any organ is the blood flow through the organ. In this regard, some investigators favor the opinion that blood volume in the adult remains unaltered as body weight increases with fat deposition, and therefore blood volume and body weight correlate poorly in adults and should be well correlated with lean body weight (3, 4).The study reported here was undertaken in order to obtain an estimate of the range of circulating red cell volume in men and women having differing degrees of obesity in an attempt to assess the relative importance of obesity tissue as an "organ" associated with a "fixed " quantity of blood. METHODS