A major aspect of the proposed research is the use of a surface analytical technique, secondary ion mass spectrometry (SIMS), for measurements of isotope ratios. This technique which requires little sample preparation, allows rapid processing in microliter volumes of samples while offering better sensitivity and precision than either neutron activation analysis or mass spectrometry of metal chelates, the methods that have been used in the past. The procedure will be compared to thermal ionization (TIMS), at present regarded the most accurate and sensitive method for isotope ratio measurements but with relatively low sample-throughput. The proposed work involves 3 stages. 1. Establishment and validity tests of SIMS for the analysis of stable Mg and Ca isotopes in testmeals, urine, feces and plasma. 2. Application of this technique to the analysis of samples enriched with stable 26Mg and 44Ca from two human studies already completed and 3. Two additional human studies to be carried out with stable isotopes of Ca and Mg respectively. The latter are planned to take advantage of the special capability of SIMS for isotope analyses in micro samples and hence its suitability for isotope detection in plasma. Stable isotope methods currently in use are applied primarily to fecal analyses. The analyses planned in the proposed study will be applied to determinations of true Ca and Mg absorption and endogenous excretion. In addition, plasma stable isotope disappearance levels will be followed for the determination of exchangeable Ca and Mg. Kinetic measurements with Ca stable isotopes will be considerably more extensive than those possible with Mg isotopes which have relatively high natural abundances. The proposed research will define the scope and limits of the isotopes of both elements relative to their respective radioisotopes. They will provide baseline data for realistic protocols for future use of stable isotopes in research on Ca and Mg metabolism.