Technetium (99mTc)-diethylene triamine pentaacetic acid (DTPA) hydrated clearance studies are accurate for determining GFR but require special facilities for handling and measuring samples. We investigated the potential of a non-radioactive paramagnetic analog, Gadolinium (Gd)-diethylene triamine pentaacetic acid (DTPA), an approved NMR contrast agent, as a glomerular filtration marker. Instead of relying on the radioactivity of technetium, this test is based on the fact that gadolinium induces alterations in the NMR T1 relaxation times in blood and urine samples. Ninety patients underwent simultaneous determinations of GFR using 1 mCi of Tc-DTPA and 0.05 mmol/kg Gd-DTPA (Berlex Labs) IV. The patients were hydrated with oral and intravenous fluid. Following a one hour equilibrium period, three or four consecutive urine collections were obtained; plasma samples were acquired at the beginning and end of each approximately 20-minute interval. 99mTc-DTPA radioactivity was determined with a scintillation counter. T1 relaxation times were measured on a 10 MHz NMR spectrometer. These were converted to Gd-DTPA concentration by comparison with standard solutions. The Gd-DTPA derived GFR closely approximated the 99mTc-DTPA derived GFR which ranged from 15 to 147 ml/min. The equation and correlation coefficient of the regression line is y = 1.04 x -2.2, r = 0.94. Thus, Gd-DTPA is a safe, non-radioactive indicator of GFR that may provide an alternative renal clearance method for clinical studies of progressive renal disease and nephrotoxicity.