Magnetic resonance is used to investigate biodistribution aspects of dextran-coated magnetite nanoparticles (9.4 nm core diameter) in both liver and spleen from 5 minutes up to 6 months after intravenous administration of a magnetic fluid sample in female Swiss mice. Using magnetic resonance data important parameters such as the absorption half-life (t 1/2 = 12 ± 2 min in the liver and t 1/2 = 11 ± 2 min in the spleen), the peak time (1.7 ± 0.2 h in the liver and 1.9 ± 0.2 h in the spleen), and the disposition half-life of the dextran-coated magnetite nanoparticles in mice organs (t 1/2 = 70 ± 10 h in the liver and t 1/2 = 32 ± 7 h in the spleen) were assessed. In addition, light and electron microscopy showed several aspects that may be related to the iron metabolism. Microscopic analysis also revealed that although magnetite nanoparticles or iron released from them are retained in the organism for a long period of time, no morphologic alteration is induced by the intravenous administration of the magnetic fluid sample, evidencing its biocompatibility. The used tests may represent an adequate methodology for nanotoxicology evaluation.