The measurement of NMRD profiles of water protons of excised tissues containing paramagnetic metal ions is one of the few ways of determining the biochemical and biophysical state of these ions in vivo. It is of critical importance, for example, to verify that Gd, injected as Gd(DTPA) to enhance contrast in MRI, remains chelated, since free Gd ions are highly toxic. We have investigated this in the renal medulla of rabbits. Fitting the magnetization data at each field of the dispersion to a single exponential shows that Gd accumulates predominantly in the renal medulla, from which it is cleared within 18 hours, and that Gd(DTPA) introduced intravenously into rabbits is excreted as Gd(DTPA) in the urine as rotationally mobile as in neat water. Taking a larger data set at each field and fitting it to the sum of two exponentials, since the errors of the single exponential analysis were larger than for other tissues, shows that the relaxation behavior of the renal medulla, free of contrast agent, can be well-described by a single relaxation rate at 37° C. For increasing concentrations of Gd in the medulla, as determined by ICP analysis, two relaxation rates are required to account for the data, due to compartmentalization of tissue water and inhomogeneous distribution of Gd. These results, and similar data after mild mechanical disruption of renal structures, show unequivocally that the Gd in the renal medulla remains the chelate complex Gd(DTPA) and rotationally mobile, for dosages up to 300 jumoles/kg injected. © Lippincott-Raven Publishers.