The paramount problem in the therapy of brain tumors is the inability of most drugs to cross the blood-brain barrier. PLGA nanoparticles overcoated with poloxamer 188 could overcome this problem and enabled a high anti-tumoral effect against the very aggressive intracranial 101.8 glioblastoma in rats that closely resembles human grade IV glioblastomas. The basis for the transport of these particles across the blood-brain barrier appears to be adsorption of blood apolipoproteins (ApoE or ApoA-I) on the nanoparticle surface caused by the poloxamer 188-coating, followed by receptor-mediated transcytosis of the nanoparticles. The objective of the present study is the elucidation of the mechanism by which the poloxamer 188-coated nanoparticles then enter the brain tumor cells. Their intracellular fate, therefore, was investigated using the U87 human glioma cell line. The main mechanism of the PLGA nanoparticle internalization by U87 cells was clathrin-mediated endocytosis. Within 1h free doxorubicin was released from late endosomes and could reach its target site, i.e. the DNA in the nuclei without degradation, whereas the PLGA nanoparticles, which were labeled with Cy5.5, still were observed in the endo-lysosomal compartment. These results demonstrate that the underlying mechanism of action in the brain cells is by diffusive doxorubicin release from the nanoparticles rather than by their intracellular degradation.