Measurements of water proton spin relaxation enhancements (epsilon) can be used to discriminate high-affinity binding of Mn-2+ or Gd-3+ to biological membranes, from low-affinity binding. In rat liver mitochondria, epsilon b values of approx. 11 are observed upon binding of Mn-2+ to the inner membrane, while internal or low-affinity binding remains invisible to this technique. Energy-driven Mn-2+ uptake by liver mitochondria results in the subsequent decay of epsilon. Comparison of epsilon with the initial velocity of Mn-2+ uptake in rat liver mitochondria reveals a linear correlation, which holds at all temperatures between 0 degrees C and 40 degrees C, regardless of the mitochondrial protein concentration. Consequently, enhancement appears to reflect the binding of Mn-2+ to the divalent cation pump. Binding of Mn-2+ to blowfly flight muscle also results in substantial epsilon, which is associated with the glycerol-1-phosphate dehydrogenase instead of divalent cation transport. Consequently, no decay in epsilon due to uptake occurs after Mn-2+ is bound. Lanthanide ions are also bound and transported by mitochondria. Addition of Gd-3+ to pigeon heart or rat liver mitochondria results in epsilon b approximately equal to 5-6, which decays with similar kinetics in both systems. The uptake velocity of Gd-3+ in rat liver mitochondria is about 1/6 the rate with which Mn-2+ is transported. Lanthanides also diminish epsilon due to the addition of Mn-2+, and greatly retard the Mn-2+ uptake kinetics. The presence of carbonylcyanide-p-trifluoromethoxyphenylhydrazone depresses epsilon upon addition of Mn-2+ or Gd-3+ and also uncouples energy-driven uptake. On the other hand, prolonged anaerobic incubation in the presence of antimycin and rotenone exhausts the mitochondria of their energy stores, blocks the uptake of Mn-2+, but does not affect epsilon significantly. Evidently, the uncoupler-induced disappearance of divalent cation binding sites is not the result of "de-energization". Measurements of epsilon at several NMR frequencies indicate a correlation time (tau b) for carrier-bound Mn-2+ in rat liver mitochondria between 20 ns and 4 ns as one varies the temperature between 10 degrees C and 30 degrees C. The 13 Kcal/mole activation energy for tau b suggests that the 11 ns time constant at room temperature represents the movement of the Mn-11-carrier comples. On the other hand, tau b is probably approx. 100 times too short to represent the rotational motion of a carrier protein. Apparently, Mn-2+ binds to a small arm of the carrier which moves independent