Manganese is a powerful paramagnetic material and potential NMR contrast agent. It drastically affects the NMR properties of solutions and tissues and is less toxic than most other transition elements. It also possesses some unusual and advantageous features; it alters T1 and T2 to different degrees, and it can bind to macromolecules to become even more effective at reducing proton relaxation times. The dose dependence of tissue relaxation rate increases has been measured in mice, and proton relaxation enhancement ratios that describe binding effects have been evaluated. These ratios imply that a tenfold reduction in manganese dose is achievable when the ion binds to intracellular components, and it is demonstrated that such binding effects can be a major factor in the efficacy of contrast enhancement. The effect of manganese on the ratio T1/T2 is dose dependent so that lower doses may be more useful for some imaging techniques. The postmortem time course of relaxation times in organs containing manganese varies between organs and with manganese content, and demonstrates that the relationship between tissue relaxation enhancement and metal content is not a simple correlation with concentration since large variations in T1 and T2 can occur even when metal and water content are fixed.