Data were obtained for the retention of Cs137 in the rat over a period of 200 days following a single administration; and data showing the buildup of Cs137 during a period of 300 days of chronic feeding. Predictions of the chronic buildup results, based on the single administration data, were in close agreement with the results of the chronic feeding experiment. The highest equilibrium Cs137 concentration (10 percent of the average daily intake per gram) was observed in muscle. Retention of cesium in muscle was closely approximated by a single exponential function with a biological half life of 13 days. There was no evidence, in any tissue, for retention of Cs137 with a half life longer than 17 days.