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The in vivo pharmacokinetics and the biodistribution of superparamagnetic iron-oxide particles (AMI25, Advanced Magnetics, Cambridge, MA) were investigated in anesthetized rats. Four different dose concentrations, ranging from 49.8 to 408.9 mumol of Fe (or 2.78-22.84 mg Fe) per kilogram, radiolabeled with 6.0 microCi of 59Fe-AMI25 were injected intravenously into 18 rats. The radioactivity cleared from the circulation with a fast component with a half-life of approximately 10 minutes and a slower component with a half-life of 92 minutes. Both half-lives were independent of the injected dose (ID) in the range of 105.4-408.9 mumol (5.89-22.84 mg) Fe/kg. The relative uptake in the liver, spleen, and kidneys was 57%, 2.9%, and 2.0% of the ID, respectively. At a dose of 52.1 mumol (2.91 mg) of Fe/kg, the relative concentration of iron significantly increased in the liver and decreased in the blood. Within the kidney, autoradiography showed that the iron was selectively taken up by the cortex. In the kidney, a concentration of 0.23 mumol (0.013 mg) Fe/g resulted in a 30% reduction in image intensity in a single echo magnetic resonance image obtained using a spin-echo sequence and an echo time of 70 ms.