658 COHN, ROSOFF, GUSMANO AND SPENCER chronic exposure was also noted by Rundo (3), who obtained values from 48 to 126 days and a meanof 87 days. These values can be compared to the mean value of 135 days as reported by Richmondet al. (4) and 115 days as reported by McNeil and Green (5). The wide variations in biological half-life reported may be due to the effects of age, or possibly metabolic variations or differences in diets of the subjects studied. In the present study the ages of the subjects were from 45 to 73 years, whereas in the other studies the subjects were generally young males. However, even in dogs of the same age and nutritional background, widespread differences in the Cs!3* turnoverrate still occur (6). A single exponential term was sufficient to describe the data for the longest study in the present series, 343 days. Richmond et al. (4) in a study covering a period of 940 days also found that a single exponential term satisfactorily de- seribed the data. On the basis of these findings, it would appear that the data may be extrapolated over considerably longer periods of time. These findings also suggest that Cs!*" is excreted primarily from one large compartment or, alternatively, is quite uniformly distributed throughout the body, as discussed in Part L. It will be of interest to investigate, in future studies, the effects of such factors as uge, diet, and metabolic disorders on the turnover rate, inasmuch as the present data suggest possible variations in the turnover on the basis of these factors. Recetvep: Deeember 5, 1962 REFERENCES 1.8. H. Cons, S. W Lippincott, E. P. Cronkite, and P. G. Reizexsteix, Application of whole-body gamma spectrometry to clinical tracer studies. Proc. Whole Body Counting Symposium, AEA. Vienna pp. 469-486 (1962). 2. K. Linty, Cesium 137 burdens in Swedish Laplanders and reindcer. Acta Radiol. 56, 237 (1961). 3. J. Runpo, Proc. Whole Body Counting Symposium, 1.A.E.A., Vienna p. 362 (1962). 4. C.R. Ricumonp, J. E. Furcuser, and W. H. LancuHam, Long term retention of radiocesium by man. Health Phys. 8, 201 (1962). 3d. K.G. McNett and R. M. Green, The effective half-lives of Cs and I in the body. Can. J. O. M Phys. 37, 528 (1959). oN. Noto, D. N. Tarver, and C. L. Comar, Cs” dose measurements in tissue. II. Health Phys. 8, 217 (1962).