The aim of the present study was to evaluate the tumor accumulation of radiolabeled long-circulating poly(ethylene glycol) (PEG)-coated hexadecylcyanoacrylate nanospheres and non-PEG-coated hexadecylcyanoacrylate nanospheres (used as control), after intravenous injection in Fischer rats bearing intracerebrally well established 9L gliosarcoma. Both types of nanospheres showed an accumulation with a retention effect in the 9L tumor. However, long-circulating nanospheres concentrated 3.1 times higher in the gliosarcoma, compared with non-PEG-coated nanospheres. The tumor-to-brain ratio of pegylated nanospheres was found to be 11, which was in accordance with the ratios reported for other carriers tested for brain tumor targeting such as long-circulating liposomes or labels for magnetic resonance imaging. In addition, a 4- to 8-fold higher accumulation of the PEG-coated carriers was observed in normal brain regions, when compared with control nanospheres. Using a simplified pharmacokinetic model, two different mechanisms were proposed to explain this higher concentration of PEG-coated nanospheres in a tumoral brain. 1) in the 9L tumor, the preferential accumulation of pegylated nanospheres was attributable to their slower plasma clearance, relative to control nanospheres. Diffusion/convection was the proposed mechanism for extravasation of the nanospheres in the 9L interstitium, across the altered blood-brain barrier. 2) In addition, PEG-coated nanospheres displayed an affinity with the brain endothelial cells (normal brain region), which may not be considered as the result of a simple diffusion/convection process. The exact underlying mechanism of such affinity deserves further investigation, since it was observed to be as important as specific interactions described for immunoliposomes with the blood-brain barrier.