Rat plasma distribution data obtained following IV administration of 99mTc(DTPA) alone or after co-administration of 99mTc(DTPA) and 153Gd-labeled Gd(DTPA)2- at 0.001, 0.1, and 1.0 mmol Gd/kg were evaluated using compartmental modeling techniques. A three-compartment open model was found to fit the data significantly better (P less than 0.01) than a two- or four-compartment open model. This model incorporates and links the plasma and urine data and includes a delay to account for the transit time through the kidneys/ureters. The two nonplasma compartments of the model were assumed to be related to rapidly and slowly equilibrating tissues. Tc(DTPA) and Gd(DTPA)2- had nearly identical pharmacokinetic profiles in plasma and the rate constants were essentially the same. No significant dose dependent pharmacokinetic differences were found for the range of Gd(DTPA)2- doses tested. Simulations of the proposed three-compartment model were used to generate concentration-time curves for each of the three compartments.