The proposed research is intended to pursue the investigation of biological ion exchange resins. Experiments are aimed at achieving a quantitative theory of biological ion selectivity through the use of NMR. The question of single ion activities in "fixed charge" systems will be resolved by the penetration of ion exchange resin beads with ion specific microelectrodes. Other studies include investigation of the interdependence of K39 NMR relaxations and tissue water and the study of K39 NMR biologic tissues in general.