[en] The decay characteristics of ^90Sr - ^90Y ensure that the mother and daughter nuclides exist in radioactive equilibrium, unless they get discriminated on the basis of their chemical properties, as it happens during metabolism. Although bone is the ultimate organ of deposition, the two nuclides arrive at this target organ over different biokinetic pathways. As ^90Y is not excreted, it goes through transient deposition in the liver before being secondarily deposited in bone. This leads to a temporary radioactive excess of ^90Y in bone. Since the decay energy of ^90Y is by a factor of about 4 higher than that of ^90Sr, the initial radiation burden to the bone marrow is primarily due to ^90Y. This was estimated in rats by implanting Li_F thermoluminescence dosimeters (TLD) in the marrow cavity of the femur. By calibrating the TLD against a known source of ^90Sr - ^90Y, the absorbed dose rates and cumulative doses were determined as a function of time after incorporation. Two routes of administration were employed and their influence on the radiation burden is also shown. (author)