A pharmacokinetic model was developed for the renal imaging agent 99mTc-DMSA in anesthetized rats, which incorporated data from serial measurements of blood and urine simultaneously with dynamic images obtained over an 8-h period. Animals which received a 10 mg/kg dose of unlabeled DMSA immediately before 99mTc-DMSA injection had a significantly reduced kidney accumulation and greater urinary elimination of 99mTc than animals which received the radiopharmaceutical alone. The kidney clearance was also significantly lower in rats receiving unlabeled DMSA, but no significant difference was determined between the urine clearance estimates of the two animal groups. Because the increase in the amount eliminated in the urine was not coupled with a significant change in urine clearance, it would appear that unlabeled DMSA saturated the kidney uptake mechanism(s) of 99mTc-DMSA without modifying the urinary clearance process. This interpretation is consistent with the hypothesis that renal handling of 99mTc-DMSA is governed by both glomerular filtration and peritubular capillary uptake. The simultaneous acquisition of blood, urine and non invasive image data allows for a comprehensive and informative model of the physiological disposition of 99mTc-DMSA.