Twenty-three mature Sprague-Dawley male and female rats were simultaneously injected with trace quantities of [153Gd]DTPA and [99mTc]DTPA and 0.5 mmol/kg of nonradioactive gadolinium DTPA. Rats were killed at 1 min, 5 min, 10 min, 15 min, and 30 min after the intracardiac bolus injection. The heart, lungs, liver, brain, kidney, and blood were excised and counted in a well-counter to determine the amount of the injected material in each organ and blood. In order for the percent of total injected activity to be determined, a technique was developed which allowed discrimination of the 140 keV gamma-ray of 99mTc from sum peaks of 153Gd when the latter is counted in a well-counter with 4 pi geometry. Although the distribution of the two DTPA compounds was qualitatively similar, statistical analysis indicated that the amount of 99mTc deposited in the lungs was higher than 153Gd (p = 0.03), the amount of 99mTc deposited in the kidneys was lower than 153Gd (p = 0.0004) and the amount of 99mTc in the blood was higher than 153Gd (p = 0.0022). This may be due to the greater binding of [99mTc]DTPA or its minor impurities to plasma proteins.