Balance studies of Sr90 have been previously performed in this laboratory under controlled dietary conditions in normal young adults (1). In these studies, the Sr90 intake was relatively high, since the diet was supplemented with milk. Tlle high fecal excretion of Sr90 indicated that the major portion of the ingested Sr90 passed unabsorbed through the intestine. The true absorption and the endogenous excretion of Sr90 could not be determined accurately, however, because of the previous intake of Sr90. Subsequently, a similar investigation was carried out during low calcium and low Sr90 intake in a group of young healthy adults of similar age group in order to obtain information on the effect of the previous Sr90 intake on the urinary and intestinal Sr90 excretion and on the comparative intestinal absorption of Sr90 during low and high Sr90 intake. The data obtained in the studies performed during low Sr90 intake are reported in this communication.