This study explored the possibility of utilizing iron oxide nanoparticles as a drug delivery vehicle for minimally invasive, MRI-monitored magnetic targeting of brain tumors. In vitro determined hydrodynamic diameter of approximately 100 nm, saturation magnetization of 94 emicro/g Fe and T2 relaxivity of 43 s(-1)mm(-)(1) of the nanoparticles suggested their applicability for this purpose. In vivo effect of magnetic targeting on the extent and selectivity of nanoparticle accumulation in tumors of rats harboring orthotopic 9L-gliosarcomas was quantified with MRI. Animals were intravenously injected with nanoparticles (12 mg Fe/kg) under a magnetic field density of 0 T (control) or 0.4 T (experimental) applied for 30 min. MR images were acquired prior to administration of nanoparticles and immediately after magnetic targeting at 1h intervals for 4h. Image analysis revealed that magnetic targeting induced a 5-fold increase in the total glioma exposure to magnetic nanoparticles over non-targeted tumors (p=0.005) and a 3.6-fold enhancement in the target selectivity index of nanoparticle accumulation in glioma over the normal brain (p=0.025). In conclusion, accumulation of iron oxide nanoparticles in gliosarcomas can be significantly enhanced by magnetic targeting and successfully quantified by MR imaging. Hence, these nanoparticles appear to be a promising vehicle for glioma-targeted drug delivery.