It is well established that the longitudinal magnetic relaxation rate of solvent water protons, 1/T1, increases markedly in homogeneous protein solutions as the magnetic field is reduced well below the traditional NMR range. For a 5% solution of protein of 10(5) Da, for example, 1/T1 increases from about 50% above the pure water rate of 20 MHz to five times the water rate at 0.01 MHz. For tissue, including blood, the behavior is similar. Data for blood show that extracellular water has ready access to the hemoglobin inside red blood cells, which causes the enhanced relaxation. The extent to which cell water can sample the spatial structure of soft tissue, and how this structure influences relaxation rates, is as yet unknown. Nonetheless, the relaxation data for tissue can be accommodated within the conceptual framework developed earlier for analyzing homogeneous solutions of diamagnetic proteins. The variation of 1/T1 with field differs among tissues, and its magnitude at a given field can vary by more than a factor of three, far more than does the water content of the tissues. Solute complexes of paramagnetic ions with macromolecules, which increase the relaxation rates of solvent protons, can be introduced intravenously in tissue. They are known to accumulate in specific organs, and therefore have potential utility as contrast-enhancing agents in NMR imaging. Mn2+ and Gd3+, for example, produce characteristic dependences of 1/T1 on magnetic field that vary with the chemical state of the agent. The possibility exists, therefore, for monitoring the in vivo biochemistry of these agents.